This should have been added in the HTTP/3 commits[2][3], but it was my
understanding that it was not supported by Hetzner DNS. It apparently is
supported but not documented.

Cloudflare has a blog post[4] explaining how this speeds up HTTP/3
negotiation. Basically, the clients can connect over HTTP/3 right away,
rather than having to connect with an older protocol (probably HTTP/2 in
our case) and then upgrade to HTTP/3 (based on the Alt-Svc header).

Our domains are HSTS preloaded[1], so it would not speed up HTTPS
negotiation in most cases.

[1] https://datatracker.ietf.org/doc/html/rfc9460
[2] 8dfa7e8c ("nginx: Add plumbing for enabling HTTP/3 conditionally")
[3] 28e0f03c ("Enable HTTP/3 for {,aur.,wiki.}archlinux.org")
[4] https://blog.cloudflare.com/speeding-up-https-and-http-3-negotiation-with-dns
[5] https://hstspreload.org/

Ref #606