treewide: drop Qt5 versions of Plasma, Gear, Deepin, Maui, friends

[K900]

TODO:

• ensure everything deleted has aliases
• add asserts for removed nixos options
• write release notes
• fix things people scream about

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[K900]

This is not actually ready, but undrafting so it pings people.

[emilazy]

You know me; I love removing things.

[spiage]

@K900 Hi!

Is it OK?

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: attribute 'kwayland-integration' missing
       at /nix/store/40hpjqbfyc4ghg1cql4m5gplnj83xy0b-source/nixos/modules/services/desktop-managers/plasma6.nix:186:9:
          185|         plasma-integration.qt5
          186|         pkgs.plasma5Packages.kwayland-integration
             |         ^
          187|         (
Traceback (most recent call last):

[K900]

Ugh. Hold on...

[spiage]

OK, it is not hurd for me )

[trofi]

Removal of nixos/tests/plasma-bigscreen.nix broke a few references in master:

$ nix build --no-link -f. nixosTests.plasma-bigscreen
error:
       … while evaluating a branch condition
         at nixos/tests/all-tests.nix:115:9:
          114|         tree:
          115|         if tree ? recurseForDerivations && tree.recurseForDerivations then
             |         ^
          116|           mapAttrs (k: findTests) (builtins.removeAttrs tree [ "recurseForDerivations" ])

       … in the left operand of the AND (&&) operator
         at nixos/tests/all-tests.nix:115:41:
          114|         tree:
          115|         if tree ? recurseForDerivations && tree.recurseForDerivations then
             |                                         ^
          116|           mapAttrs (k: findTests) (builtins.removeAttrs tree [ "recurseForDerivations" ])

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: path 'nixos/tests/plasma-bigscreen.nix' does not exist

[vcunat]

deepin is also de-facto dying on NixOS 25.05 right now:
https://hydra.nixos.org/eval/1818609?compare=1818069&filter=deepin

[wineee]

deepin is also de-facto dying on NixOS 25.05 right now:

Some development libraries in deepin use Qt private APIs, and even a minor update to Qt 6.9.2 could break compatibility;
Arch Linux already has a patched fix：https://gitlab.archlinux.org/archlinux/packaging/packages/deepin-qt6integration/-/blob/main/qt-6.9.2.patch?ref_type=heads
However, I wonder whether continuing to maintain dde on NixOS 25.05 still makes sense.

[vcunat]

It's unpleasant that the regression comes to stable NixOS release in the middle of its support period, but I have no idea how many people rely on this. (I guess we'll find out now.)