add deprecation note to gRPC ServerMsg.topic

pbx/model.proto

@@ -478,6 +478,7 @@ message ServerMsg {
 		ServerMeta meta = 4;
 		ServerInfo info = 5;
 	}
+	// DEPRECATED. Will be removed soon.
 	// When response is sent to Root, send internal topic name too.
 	string topic = 6;
 }