Update the macOS version used in GitHub Actions (again)

[guihkx]

This is was actually done recently in PR #681, but now we kind of have to do it again because the build job for Homebrew (Qt 6) is building and installing Qt 6 from source, which take hours...

Homebrew's website lists macOS 13 as the minimum supported version (and we currently use macOS 12 on GitHub Actions), so we might use that version for all macOS jobs...