send_join response: get create event from state, not auth_chain (#12005)

msc3706 proposes changing the `/send_join` response:

> Any events returned within `state` can be omitted from `auth_chain`.

Currently, we rely on `m.room.create` being returned in `auth_chain`, but since
the `m.room.create` event must necessarily be part of the state, the above
change will break this.

In short, let's look for `m.room.create` in `state` rather than `auth_chain`.

synapse/handlers/federation_event.py

@@ -420,7 +420,7 @@ class FederationEventHandler:
             SynapseError if the response is in some way invalid.
         """
         create_event = None
-        for e in auth_events:
+        for e in state:
             if (e.type, e.state_key) == (EventTypes.Create, ""):
                 create_event = e
                 break