`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`.