Send the m.room.create stripped event with invites (support MSC1772). (#9966)

MSC1772 specifies the m.room.create event should be sent as part
of the invite_state. This was done optionally behind an experimental
flag, but is now done by default due to MSC1772 being approved.

docs/sample_config.yaml

@@ -1521,6 +1521,7 @@ room_prejoin_state:
    # - m.room.avatar
    # - m.room.encryption
    # - m.room.name
+   # - m.room.create
    #
    # Uncomment the following to disable these defaults (so that only the event
    # types listed in 'additional_event_types' are shared). Defaults to 'false'.