Add synonyms feature for test clusters

For snapshots builds we automatically enable all feature flags,
but for release builds they need to be explicitly added to
test clusters for tests.
This PR does it for synonyms feature.

Closes elastic#96641, elastic#97177

rest-api-spec/build.gradle

@@ -1,3 +1,5 @@
+import org.elasticsearch.gradle.Version
+
 apply plugin: 'elasticsearch.build'
 apply plugin: 'elasticsearch.publish'
 apply plugin: 'elasticsearch.rest-resources'
@@ -37,6 +39,10 @@ dependencies {
   clusterModules project(":modules:reindex")
 }
 
+testClusters.configureEach {
+  requiresFeature 'es.synonyms_feature_flag_enabled', Version.fromString("8.9.0")
+}
+
 tasks.named("yamlRestTestV7CompatTransform").configure { task ->
 
   task.skipTestsByFilePattern("**/cat*/*.yml", "Cat API are meant to be consumed by humans, so will not be supported by Compatible REST API")