Set visibility of failure_store param of Rollover API to feature_flag (#107061)

rest-api-spec/src/main/resources/rest-api-spec/api/indices.rollover.json

@@ -66,7 +66,9 @@
       },
       "failure_store":{
         "type":"boolean",
-        "description":"If set to true, the rollover action will be applied on the failure store of the data stream."
+        "description":"If set to true, the rollover action will be applied on the failure store of the data stream.",
+        "visibility": "feature_flag",
+        "feature_flag": "es.failure_store_feature_flag_enabled"
       }
     },
     "body":{