github_runner_matrix: decouple macOS versions from MacOSVersion#supported_release?

[carlocab]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

See discussion at #18296.

[Bo98]

Thanks!

[MikeMcQuaid]

Great, thanks for this @carlocab!