This removes most of the code used for polylith/API mode.
This removes the `/api` internal endpoints entirely.
Binary size change roughly 5%:
```
51437560 Feb 13 10:15 dendrite-monolith-server # old
48759008 Feb 13 10:15 dendrite-monolith-server # new
```
* Add restrications for open registration
* Make enable open registration a parameter
* Enable registration for CI
* Update error message
* Shuffle things around a bit
* Add a warning at every startup just to be extra annoying
* Ignore shared secret when warning about open registration, since it's not strictly required when it is set if registration is otherwise enabled
* Make CI happy?
* Add missing parameter; try new parameter in upgrade-test
Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>