fix compatibility

Author: lkts

rest-api-spec/build.gradle

@@ -98,4 +98,5 @@ tasks.named("yamlRestCompatTestTransform").configure ({ task ->
   task.skipTest("index/91_metrics_no_subobjects/Metrics object indexing with synthetic source", "_source.mode mapping attribute is no-op since 9.0.0")
   task.skipTest("index/91_metrics_no_subobjects/Root without subobjects with synthetic source", "_source.mode mapping attribute is no-op since 9.0.0")
   task.skipTest("logsdb/10_settings/routing path allowed in logs mode with routing on sort fields", "Unknown feature routing.logsb_route_on_sort_fields")
+  task.skipTest("synthetic_source with copy_to inside nested object", "temporary until backported")
 })