nixos/gnome & nixos/gdm: migration out of x11

[s0me1newithhand7s]

Reasons:

since DEs like KDE Plasma 6, GNOME and COMSIC are not designed to be x11-exclusive putting them under services.xserver is misleading; to confirm my point - this PR and this PR exist.
also, GNOME's default is wayland, and x11 sessions is an option, that could be dropped in future.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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
• Fits CONTRIBUTING.md.

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[stellessia]

Let's freaking go!!! I've waited for that change since I rolled into NixOS. Thank you, my dear friend

[jtojnar]

Thanks, for doing this.

The change looks reasonable at first glance, though tests should probably be updated as well.

I am currently focusing on GNOME 48 upgrade, will take a deeper look after that is done.