Update systemd.nix

This commit:

• replaces dependencies on the obsolete¹ acme-certificates.target and acme-selfsigned-certificates.target with acme-finished-${cfg.fqdn}.target
• removes the unnecessary mailserver-certificates.target
• brings a few trivial syntax fixes

[1] see https://nixos.org/manual/nixos/stable/release-notes.html#sec-release-19.09-incompatibilities and https://github.com/NixOS/nixpkgs/blob/release-20.09/nixos/modules/security/acme.nix#L663-L668