Use dedicated get_local_users_in_room to find local users when calculating join_authorised_via_users_server of a /make_join request (#13606)

Use dedicated `get_local_users_in_room` to find local users when calculating `join_authorised_via_users_server` ("the authorising user for joining a restricted room") of a `/make_join` request.

Found while working on #13575 (comment) but it's not related.

Author: MadLittleMods

changelog.d/13606.misc

@@ -0,0 +1 @@
+Use dedicated `get_local_users_in_room(room_id)` function to find local users when calculating `join_authorised_via_users_server` of a `/make_join` request.

synapse/handlers/event_auth.py

@@ -129,12 +129,9 @@ async def get_user_which_could_invite(
         else:
             users = {}
 
-        # Find the user with the highest power level.
-        users_in_room = await self._store.get_users_in_room(room_id)
-        # Only interested in local users.
-        local_users_in_room = [
-            u for u in users_in_room if get_domain_from_id(u) == self._server_name
-        ]
+        # Find the user with the highest power level (only interested in local
+        # users).
+        local_users_in_room = await self._store.get_local_users_in_room(room_id)
         chosen_user = max(
             local_users_in_room,
             key=lambda user: users.get(user, users_default_level),