CI: test on particular versions of macos, not macos-latest

[dannywillems]

Based on https://github.com/actions/runner-images/tree/main, other runners are available for macOS.
We want to update the CI to run specific versions of macOS, and not only the latest.
The CI should be updated to support macos-14, macos-15. macos-13 and macos-latest.

The website should be updated accordingly to mention which versions of MacOS we support in the CI, and which one we consider supporting.