Motivation for this change

I run Postfix on my workstation as a smarthost, where it only ever talks to my SMTP server. Because I know it'll only ever connect to this server, and because I know this server supports TLS, I'd like to set smtp_tls_security_level to "encrypt" so Postfix won't fall back to an unencrypted connection.

I’ve also removed services.postfix.useDane, because now that smtp_tls_security_level is using mkDefault, and therefore can
be overridden, there's no need for an option for overriding it to a specific value.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.