Make use of `macos15-intel` runners

[cfergeau]

The macos13 github runners are deprecated.
The newer runners using Apple silicon hardware are not that useful to us as they don’t support virtualization.
However, github now offers macos15-intel runners which we can use as a replacement for the macos13 runners.

[cfergeau]

See https://docs.github.com/en/actions/reference/runners/github-hosted-runners#standard-github-hosted-runners-for-public-repositories
and
actions/runner-images#13045