ADMIN: Update MacOS GitHub action runners

[tturocy]

Two inter-related items:

1. We're still using the MacOS-13 runner, which is being deprecated. We should remove this and add a MacOS-15 one.
2. We don't really need to be building artefacts for the regular runs anymore. We only need the artefact for the build that is triggered on release, because that's what creates the GUI executable that is linked statically against wxWidgets. As sometimes these builds fail at the artefact stage due to resource unavailability, we should probably just drop this while making these changes.