Ubuntu CI fails during dependency installation after runner upgrade #567

Both Ubuntu GCC and Clang jobs on [PR #566](#566) fail before compilation:

Package 'libqt6websockets6-dev' has no installation candidate

ubuntu-latest now selects Ubuntu 26.04.1, where the package has been replaced by qt6-websockets-dev. GitHub began rolling out this runner change on 19 October, explaining why earlier PR tests passed.

Suggested fix: pin the test matrix to ubuntu-24.04, including its exclusion entry, or update and validate the dependencies for Ubuntu 26.04. Pinning would prevent future runner upgrades from unexpectedly breaking dependency installation.

[Failed job log](https://github.com/MUME/MMapper/actions/runs/40603501209/job/122537178297)