Enabling migration reindex for XPackRestIT (#118359) (#119241)

This enables the feature flag for migration reindexing for XPackRestIT.
Closes #118272 Closes #118273  Closes #118274

Author: masseyke

test/test-clusters/src/main/java/org/elasticsearch/test/cluster/FeatureFlag.java

@@ -19,7 +19,8 @@ public enum FeatureFlag {
     TIME_SERIES_MODE("es.index_mode_feature_flag_registered=true", Version.fromString("8.0.0"), null),
     FAILURE_STORE_ENABLED("es.failure_store_feature_flag_enabled=true", Version.fromString("8.12.0"), null),
     SUB_OBJECTS_AUTO_ENABLED("es.sub_objects_auto_feature_flag_enabled=true", Version.fromString("8.16.0"), null),
-    INFERENCE_UNIFIED_API_ENABLED("es.inference_unified_feature_flag_enabled=true", Version.fromString("8.18.0"), null);
+    INFERENCE_UNIFIED_API_ENABLED("es.inference_unified_feature_flag_enabled=true", Version.fromString("8.18.0"), null),
+    MIGRATION_REINDEX_ENABLED("es.reindex_data_stream_feature_flag_enabled=true", Version.fromString("8.18.0"), null);
 
     public final String systemProperty;
     public final Version from;