Consider enabling HTTP/3

It has been enabled in the nginx package nearly three months ago, so it should work?

Plan:

1. Add relevant plumbing to the nginx role (enable HTTP/3, logging of HTTP version ($server_protocol), Alt-Svc header)
2. Enable it for a few services with a low Alt-Svc max-age
3. Sit tight and assess
4. Enable it for more services

mentioned in issue #608 (closed)

mentioned in commit 28e0f03c

mentioned in commit 8dfa7e8c

mentioned in merge request !872 (merged)

mentioned in commit f5475cae

mentioned in commit 84421e62