Serving partial join responses is no longer experimental. They will only be served under the stable identifier if the the undocumented config flag experimental.msc3706_enabled is set to true.

Synapse continues to request a partial join only if the undocumented config flag experimental.faster_joins is set to true; this setting remains present and unaffected.

Fix https://github.com/matrix-org/synapse/issues/14390

 - Client API: `/_matrix/client/unstable/org.matrix.msc3030/rooms/<roomID>/timestamp_to_event?ts=<timestamp>&dir=<direction>` -> `/_matrix/client/v1/rooms/<roomID>/timestamp_to_event?ts=<timestamp>&dir=<direction>`
 - Federation API: `/_matrix/federation/unstable/org.matrix.msc3030/timestamp_to_event/<roomID>?ts=<timestamp>&dir=<direction>` -> `/_matrix/federation/v1/timestamp_to_event/<roomID>?ts=<timestamp>&dir=<direction>`

Complement test changes: https://github.com/matrix-org/complement/pull/559

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

Fix Complement `TestCanRegisterAdmin` with workers, by adding Complement's shared registration secret. (#12819)

Fixes matrix-org/complement#330 (or it will, once we remove the old files).

It's not quite a lift-and-shift: I've also taken the opportunity to get rid of the custom CA that we used to use to sign the TLS certs, which has been superceded by the CA exposed by Complement.