* Also use stable name in SendJoinResponse struct

follow-up to #14832

* Changelog

* Fix a rename I missed

* Run black

* Update synapse/federation/federation_client.py

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>