matrix.archlinux.org: Configure static DNS via networking role

Hetzner DNS has been delaying many responses for 5 seconds, causing
outgoing federation work to pile up, almost running into OOM before we
noticed.

I don't know if were being throttled because federation makes a *lot* of
requests. Anyway, using Cloudflare DNS seems to solve it.

Enable DNSOverTLS for this because we can.