Co-authored-by: David Robertson <david.m.robertson1@gmail.com>

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

Add an option allowing users to use their password to reauthenticate even though password authentication is disabled. (#12883)

Makes it so that groups/communities no longer exist from a user-POV. E.g. we remove:

* All API endpoints (including Client-Server, Server-Server, and admin).
* Documented configuration options (and the experimental flag, which is now unused).
* Special handling during room upgrades.
* The `groups` section of the `/sync` response.

Co-authored-by: Matthew Hodgson <matthew@matrix.org>

* Add mau_appservice_trial_days

* Add a test

* Tweaks

* changelog

* Ensure we sync after the delay

* Fix types

* Add config statement

* Fix test

* Reinstate logging that got removed

* Fix feature name

* Update config_documentation.md

Co-authored-by: David Robertson <davidr@element.io>

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>