Use Qt 6 for macOS workflow

[mrbean-bremen]

• current macos does no longer support these

Note that these runners are deprecated and will be removed at some time, so the CI will not work anyway then.
Maybe instead we can just remove these...
Also, at the moment there seem to be no macos-14 runners available - this may be temporary, but they are probably already in the process of being phased out.

Update:

Found this on the runners site:

GitHub Actions announcing the deprecation process for macOS 14 and macOS 14 arm64. While the images are being deprecated, You may experience longer queue times during peak usage hours. Deprecation will begin on July 6th, 2026 and the images will be fully unsupported by November 2nd, 2026 for GitHub Actions and Azure DevOps.

So it probably makes sense to remove the macos/Qt 5.x workflow right away. I'll update the PR to do that instead of using an older runner.
Edit: Done.

[usiems]

It seems with this we will have no CI support for macOS anymore. I will approve this merge request, since the old checks simply don't work anymore, but is someone willing to try to get this running again for some macOS version?

[mrbean-bremen]

It seems with this we will have no CI support for macOS anymore

Ah - I was under the impression that we have macOS builds in the other job, but I was obviously wrong. Instead of removing the job, we can try to change it to use Qt 6.x - that should be supported. I'll try it out...

[mrbean-bremen]

I've changed the PR to use a current Qt version for testing on macOS. Building it took ages - hopefully this will be improved next time by ccache.