Check support room has only two users before sending a notice (#8728)

* Check support room has only two users

* Create 8728.bugfix

* Update synapse/server_notices/server_notices_manager.py

Co-authored-by: Erik Johnston <erik@matrix.org>

Co-authored-by: Erik Johnston <erik@matrix.org>

changelog.d/8728.bugfix

@@ -0,0 +1 @@
+Fix bug where the `/_synapse/admin/v1/send_server_notice` API could send notices to non-notice rooms.

synapse/server_notices/server_notices_manager.py

@@ -119,7 +119,7 @@ class ServerNoticesManager:
             # manages to invite the system user to a room, that doesn't make it
             # the server notices room.
             user_ids = await self._store.get_users_in_room(room.room_id)
-            if self.server_notices_mxid in user_ids:
+            if len(user_ids) <= 2 and self.server_notices_mxid in user_ids:
                 # we found a room which our user shares with the system notice
                 # user
                 logger.info(