Reduce invalid set type errors to WARNING

It's too big of a churn right now to actually break builds that use dangerous
types with sets. This change leaves the documentation as-is, so the only way to
use a set with a dangerous type is experimentation or having already done so.

smithy-model/src/main/java/software/amazon/smithy/model/validation/validators/TargetValidator.java

@@ -158,10 +158,11 @@ private Optional<ValidationEvent> validateSetMember(Shape shape, ShapeId target,
         }
 
         if (!VALID_SET_TARGETS.contains(targetShape.getType())) {
-            return Optional.of(error(shape, format(
+            return Optional.of(warning(shape, format(
                     "Set member targets %s, but sets can target only %s. You can model a collection of %s shapes "
                     + "by changing this shape to a list. Modeling a set of values of other types is problematic "
-                    + "to support across a wide range of programming languages.",
+                    + "to support across a wide range of programming languages. This will be upgraded to an ERROR "
+                    + "in a future release.",
                     targetShape,
                     VALID_SET_TARGETS.stream().map(ShapeType::toString).sorted().collect(Collectors.joining(", ")),
                     targetShape.getType())));

smithy-model/src/test/resources/software/amazon/smithy/model/errorfiles/validators/sets/invalid-sets.errors

@@ -1,12 +1,12 @@
-[ERROR] smithy.example#TimestampSet: Set member targets (timestamp: `smithy.api#Timestamp`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of timestamp shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. | Target
-[ERROR] smithy.example#BooleanSet: Set member targets (boolean: `smithy.api#Boolean`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of boolean shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. | Target
-[ERROR] smithy.example#DoubleSet: Set member targets (double: `smithy.api#Double`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of double shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. | Target
-[ERROR] smithy.example#FloatSet: Set member targets (float: `smithy.api#Float`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of float shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. | Target
-[ERROR] smithy.example#DocumentSet: Set member targets (document: `smithy.api#Document`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of document shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. | Target
-[ERROR] smithy.example#ListSet: Set member targets (list: `smithy.example#StringList`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of list shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. | Target
-[ERROR] smithy.example#SetSet: Set member targets (set: `smithy.example#StringSet`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of set shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. | Target
-[ERROR] smithy.example#MapSet: Set member targets (map: `smithy.example#StringMap`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of map shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. | Target
-[ERROR] smithy.example#StructSet: Set member targets (structure: `smithy.example#StructureExample`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of structure shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. | Target
-[ERROR] smithy.example#UnionSet: Set member targets (union: `smithy.example#UnionExample`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of union shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. | Target
+[WARNING] smithy.example#TimestampSet: Set member targets (timestamp: `smithy.api#Timestamp`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of timestamp shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. This will be upgraded to an ERROR in a future release. | Target
+[WARNING] smithy.example#BooleanSet: Set member targets (boolean: `smithy.api#Boolean`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of boolean shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. This will be upgraded to an ERROR in a future release. | Target
+[WARNING] smithy.example#DoubleSet: Set member targets (double: `smithy.api#Double`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of double shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. This will be upgraded to an ERROR in a future release. | Target
+[WARNING] smithy.example#FloatSet: Set member targets (float: `smithy.api#Float`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of float shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. This will be upgraded to an ERROR in a future release. | Target
+[WARNING] smithy.example#DocumentSet: Set member targets (document: `smithy.api#Document`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of document shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. This will be upgraded to an ERROR in a future release. | Target
+[WARNING] smithy.example#ListSet: Set member targets (list: `smithy.example#StringList`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of list shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. This will be upgraded to an ERROR in a future release. | Target
+[WARNING] smithy.example#SetSet: Set member targets (set: `smithy.example#StringSet`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of set shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. This will be upgraded to an ERROR in a future release. | Target
+[WARNING] smithy.example#MapSet: Set member targets (map: `smithy.example#StringMap`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of map shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. This will be upgraded to an ERROR in a future release. | Target
+[WARNING] smithy.example#StructSet: Set member targets (structure: `smithy.example#StructureExample`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of structure shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. This will be upgraded to an ERROR in a future release. | Target
+[WARNING] smithy.example#UnionSet: Set member targets (union: `smithy.example#UnionExample`), but sets can target only bigDecimal, bigInteger, blob, byte, integer, long, short, string. You can model a collection of union shapes by changing this shape to a list. Modeling a set of values of other types is problematic to support across a wide range of programming languages. This will be upgraded to an ERROR in a future release. | Target
 [ERROR] smithy.example#StreamingBlobSet: Set member targets (blob: `smithy.example#StreamingBlob`), a shape marked with the @streaming trait. | Target
 [ERROR] smithy.example#StreamingStringSet: Set member targets (string: `smithy.example#StreamingString`), a shape marked with the @streaming trait. | Target