opencv4: drop gtk2 support for highgui

[ShadowRZ]

Part of #410814

OpenCV 4 highgui can build with GTK3 and also prefers it: https://docs.opencv.org/4.13.0/db/d05/tutorial_config_reference.html#tutorial_config_reference_highgui

Leaf impacted packages:

• apriltags: enables both GTK2 and GTK3, will actually use GTK3 at runtime so should have no loss
• howdy, linux-enable-ir-emitter: all depends on python3Packages.opencv4Full. as above the built OpenCV 4 will use GTK3, not GTK2. They also directly depends on GTK3.
• rPackages.opencv: I don't think this would require GTK2 in any way, and should work with GTK3 highgui.

--------- Impacted packages on 'x86_64-linux' ---------
5 packages updated:
apriltags howdy linux-enable-ir-emitter opencv opencv

Closes #545102

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • 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.
• Follows the automation/AI policy.