Bugfix make_room_admin fails for users that have left a private room (#10367)

Fixes: #10338

Author: dklimpel

changelog.d/10367.bugfix

@@ -0,0 +1 @@
+Bugfix `make_room_admin` fails for users that have left a private room.

synapse/rest/admin/rooms.py

@@ -462,6 +462,7 @@ def __init__(self, hs: "HomeServer"):
         super().__init__(hs)
         self.hs = hs
         self.auth = hs.get_auth()
+        self.store = hs.get_datastore()
         self.event_creation_handler = hs.get_event_creation_handler()
         self.state_handler = hs.get_state_handler()
         self.is_mine_id = hs.is_mine_id
@@ -500,7 +501,13 @@ async def on_POST(
             admin_user_id = None
 
             for admin_user in reversed(admin_users):
-                if room_state.get((EventTypes.Member, admin_user)):
+                (
+                    current_membership_type,
+                    _,
+                ) = await self.store.get_local_current_membership_for_user_in_room(
+                    admin_user, room_id
+                )
+                if current_membership_type == "join":
                     admin_user_id = admin_user
                     break