Revert 'Remove the unspecced field in the response. (#13365)' to give more time for clients to update. (#13501)

changelog.d/13501.misc

@@ -0,0 +1 @@
+Revert 'Remove the unspecced `room_id` field in the `/hierarchy` response. (#13365)' to give more time for clients to update.

synapse/handlers/room_summary.py

@@ -453,6 +453,7 @@ class RoomSummaryHandler:
                 "type": e.type,
                 "state_key": e.state_key,
                 "content": e.content,
+                "room_id": e.room_id,
                 "sender": e.sender,
                 "origin_server_ts": e.origin_server_ts,
             }