test/github_runner_matrix: prevent dropping supported macOS versions from CI

[carlocab]

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• Have you checked to ensure there aren't other open Pull Requests for the same change?
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We should bump HOMEBREW_MACOS_OLDEST_SUPPORTED first and make sure
that bump goes out into a release tag before dropping CI in
Homebrew/core. Otherwise, we end up with users getting errors about
missing bottles. See, for example, Homebrew/homebrew-core#240857.

We've made it very hard for users on OS versions newer than
HOMEBREW_MACOS_OLDEST_SUPPORTED to not use bottles (because only
bottles are supported), but that design assumes we have been building
bottles for them. We should therefore make sure that users we stop
building bottles for are always on a version older than
HOMEBREW_MACOS_OLDEST_SUPPORTED.

[MikeMcQuaid]

Makes sense, thanks!