Motivation for this change

My previous fix (now merged #80154) introduced a bug by removing apply = lib.mapAttrs (name: text: pkgs.writeText "${name}" text); causing shorewall.service to be corrupted, because X-Restart-Triggers is now set to a garbage value (the content of the files instead of their paths).
The service was still starting but many warnings of wrong syntax were emitted in its logs.

Things done

Fix Restart-Triggers in shorewall and shorewall6 services by reverting apply and using .source instead of .text.
For config files, the more correct types.lines is also used instead of types.str.

• 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.