refactor(attic): move headscale from mx to attic

Headscale is moving off the mx mailserver onto the attic cache host.
The new public URL is https://headscale.hoyer.world.

- Switch from useACMEHost = "hoyer.xyz" (mx wildcard DNS-01) to
  enableACME = true, since attic only has HTTP-01 configured.
- Move headscale port to 8081 to avoid clashing with atticd on 8080.
- Drop the 192.168.178.254 LAN nameserver from dns.nameservers.global,
  which isn't reachable from the Hetzner instance.

Operational steps still required on attic:
- Provision /var/lib/headscale/client_secret
- Migrate the headscale state DB from mx
- Point headscale.hoyer.world DNS at attic
- Update the Nextcloud OIDC client's redirect URI

systems/x86_64-linux/mx/default.nix

@@ -13,7 +13,6 @@
     ./disk-check.nix
     ./forgejo.nix
     ./hardware-configuration.nix
-    ./headscale.nix
     ./kicker.nix
     ./mailserver.nix
     ./network.nix