Enable HTTP/3 for {,aur.,wiki.}archlinux.org

They are our HTTP/3 guinea pigs for now. HTTP/3 has been enabled on
archlinux.org since 2024-07-22, so I do not expect any issues.

$http_host is changed to $host for aurweb, as HTTP/3 uses the
":authority" pseudo-header instead of the "Host" header[1][2].

[1] https://trac.nginx.org/nginx/ticket/2281
[2] https://mailman.nginx.org/pipermail/nginx-devel/2024-January/LCIUMLKCM2EBMEMTU3KXMW74AP2C4FYZ.html

Ref #606