[ML] Add _meta information to ML ILM policy

This was missed in elastic#73629

Author: droberts195

x-pack/plugin/core/src/main/resources/org/elasticsearch/xpack/core/ml/size_based_ilm_policy.json

@@ -7,5 +7,9 @@
         }
       }
     }
+  },
+  "_meta": {
+    "description": "default policy for machine learning state and stats indices",
+    "managed": true
   }
 }