nixos/wordpress: Drop old deprecated interface

[onny]

Motivation for this change

This PR drops the old deprecated interface from the Wordpress module. The former syntax
services.wordpress."hostname.local" = {} changed to services.wordpress.sites."hostname.local" = {} with NixOS 21.11.
It would be good to drop the old deprecated interface with 22.05 to cleanup the module.
The deprecation warning was introduced with 21.11 and also noted in the release notes.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[onny]

Added some release notes about the removal of the old deprecated interface

[onny]

Thank you @eonpatapon that was a good hint, fixed it :)

[happysalada]

Thank you!