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I've also taken the opportunity to get rid of the custom CA that we used to use to sign the TLS certs, which has been superceded by the CA exposed by Complement.