CI: Disable GNU toolchain

[kalenikaliaksandr]

Motivation:

• Sooner or later we will start using Swift supported only by Clang
• GCC CI runs tests visibly slower than Clang CI
• Less build issues with Clang, e.g. recent attempt to add ANGLE that fails to compile with GCC

[awesomekling]

This makes sense to me. The GCC CI is a constant source of annoyances and time-wasting, unfortunately.

[LucasChollet]

If we go down this way, we probably also want to reject GCC in ladybird.sh. People will try to build with GCC and it will result in errors the second you remove the build from CI.

[rmg-x]

Same with documentation updates to avoid an uptick in issues and/or people going to #build-problems on Discord 😅

[shlyakpavel]

Do I understand correctly that once we make Swift mandatory, we’ll be committed to using a single toolchain for a long time?

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!