Uniformize spam-checker API, part 4: port other spam-checker callbacks to return `Union[Allow, Codes]`. (#12857)

Co-authored-by: Brendan Abolivier <babolivier@matrix.org>

Missed by #12475.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Update worker docs to remove group endpoints.
* Removes an unused parameter to `ApplicationService`.
* Break dependency between media repo and groups.
* Avoid copying `m.room.related_groups` state events during room upgrades.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.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)

Removes the unstable endpoint as well as a duplicated field
which was modified during stabilization.

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.

Signed-off-by: David Teller <davidt@element.io>

Signed-off-by: jesopo <github@lolnerd.net>

Add a unique index to `state_group_edges` to prevent duplicates being accidentally introduced and the consequential impact to performance. (#12687)

Signed-off-by: Sami Olmari <sami@olmari.fi>

docbook doesn't auto-linkify links

Signed-off-by: Sean Quah <seanq@matrix.org>

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

* Update openid.md

Newer versions of keycloak returning a 404 when using the `/auth` prefix.

Related: https://github.com/matrix-org/synapse/issues/12714

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

Add the `update_user_directory_from_worker` configuration option (superseding `update_user_directory`) to allow a generic worker to be designated as the worker to update the user directory. (#12654)

Co-authored-by: Shay <hillerys@element.io>