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

host_vars/archlinux.org/misc

@@ -12,3 +12,4 @@ fail2ban_jails:
   nginx_limit_req: true
 wireguard_address: 10.0.0.1
 wireguard_public_key: 0Vx7jfWinpTPHKPxvmKtZlp3hcLebawz+vQM8EIEm1k=
+nginx_enable_http3: true

host_vars/aur.archlinux.org/misc

@@ -7,3 +7,4 @@ fail2ban_jails:
 memcached_socket: "/run/memcached/aurweb.sock"
 wireguard_address: 10.0.0.2
 wireguard_public_key: TPLeGQ7qU6ZNtcgDbEV0SSYScvK+XS5igcPdGSXo6UA=
+nginx_enable_http3: true

host_vars/wiki.archlinux.org/misc

@@ -4,3 +4,4 @@ wireguard_address: 10.0.0.22
 wireguard_public_key: bZeNWMLtyNDaFR7jjWr06nNZt/vV/OKNleV7XZZs+lc=
 nginx_extra_modules:
   - name: geoip2
+nginx_enable_http3: true

roles/aurweb/templates/nginx.d.conf.j2

@@ -140,7 +140,7 @@ server {
     location / {
         # Proxy over to aurweb's ASGI application.
         proxy_pass http://{{ aurweb_asgi_bind }};
-        proxy_set_header Host $http_host;
+        proxy_set_header Host $host;
         proxy_set_header X-Forwarded-For $remote_addr;
         proxy_set_header X-Forwarded-Proto $scheme;
         proxy_set_header X-Forwarded-Ssl on;