Disable index.mapper.dynamic index setting validation. (#109160)

This setting was removed via #25734, because the setting no longer used
since 6.0.0

However, the validation only kicked when trying to set this setting on a
closed index. Applying the setting on an open index would just work.
With severe consequences later on. For example when upgrading the
cluster, nodes would refuse to boot, because the validation would kick
in.

Relates to #96075

docs/changelog/109160.yaml

@@ -0,0 +1,5 @@
+pr: 109160
+summary: Disable `index.mapper.dynamic` index setting validation
+area: Mapping
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/index/mapper/MapperService.java

@@ -59,6 +59,8 @@
 
 public class MapperService extends AbstractIndexComponent implements Closeable {
 
+    private static final DeprecationLogger DEPRECATION_LOGGER = DeprecationLogger.getLogger(MapperService.class);
+
     /**
      * The reason why a mapping is being merged.
      */
@@ -205,7 +207,12 @@ public MapperService(
 
         if (INDEX_MAPPER_DYNAMIC_SETTING.exists(indexSettings.getSettings())
             && indexSettings.getIndexVersionCreated().onOrAfter(Version.V_7_0_0)) {
-            throw new IllegalArgumentException("Setting " + INDEX_MAPPER_DYNAMIC_SETTING.getKey() + " was removed after version 6.0.0");
+            DEPRECATION_LOGGER.warn(
+                DeprecationCategory.MAPPINGS,
+                "index.mapper.dynamic",
+                "Setting {} was removed after version 6.0.0",
+                INDEX_MAPPER_DYNAMIC_SETTING.getKey()
+            );
         }
         defaultMappingSource = "{\"_default_\":{}}";
         if (logger.isTraceEnabled()) {