Add deprecation notice for missing option

Relates #29465

Author: jimczi

docs/reference/migration/migrate_6_0/aggregations.asciidoc

@@ -63,4 +63,9 @@ If the `format` in the mappings is not compatible with the numeric input value,
 
 The execution hints `global_ordinals_hash` and `global_ordinals_low_cardinality` are deprecated and should be replaced
 by `global_ordinals` which now internally choose whether it should remap global ordinals to dense ordinals or directly use the
-segment ordinals.
+segment ordinals.
+
+==== `missing` is deprecated in the `composite` aggregation
+
+The `missing` option of the `composite` aggregation is deprecated, `missing_bucket`
+should be used instead.

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/230_composite.yml

@@ -327,8 +327,8 @@ setup:
 ---
 "Composite aggregation and array size":
   - skip:
-      version: " - 6.99.99"
-      reason:  starting in 7.0 the composite sources do not allocate arrays eagerly.
+      version: " - 6.3.99"
+      reason:  starting in 6.4 the composite sources do not allocate arrays eagerly.
 
   - do:
         search:

server/src/main/java/org/elasticsearch/search/aggregations/bucket/composite/CompositeValuesSourceBuilder.java

@@ -72,7 +72,7 @@ public abstract class CompositeValuesSourceBuilder<AB extends CompositeValuesSou
         if (in.readBoolean()) {
             this.valueType = ValueType.readFromStream(in);
         }
-        if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (in.getVersion().onOrAfter(Version.V_6_4_0)) {
             this.missingBucket = in.readBoolean();
         } else {
             this.missingBucket = false;
@@ -100,7 +100,7 @@ public final void writeTo(StreamOutput out) throws IOException {
         if (hasValueType) {
             valueType.writeTo(out);
         }
-        if (out.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (out.getVersion().onOrAfter(Version.V_6_4_0)) {
             out.writeBoolean(missingBucket);
         }
         out.writeGenericValue(missing);