We have a `HOMEBREW_MACOS_NEWEST_UNSUPPORTED` environment variable
and this is used in `MacOSVersion` to determine prerelease versions
but we don't have a way of easily determining the newest supported
macOS version.
`bump-cask-pr` contains logic that assumes the first key/value in
`MacOSVersion::SYMBOLS` is the newest macOS version but it recently
became clear that this is a prerelease version between WWDC and the
subsequent macOS release. Similarly, `dev-cmd/generate-cask-api.rb`
tries to compute the newest stable macOS version as
`HOMEBREW_MACOS_NEWEST_UNSUPPORTED.to_i - 1` and this will fail
if/when we update that variable to `"26"`, as the macOS version
before 26 is 15, not 25.
This adds a `HOMEBREW_MACOS_NEWEST_SUPPORTED` environment variable,
so we have a straightforward way of quickly identifying the newest
supported macOS version without having to make potentially unreliable
assumptions or do computations to identify the latest non-prerelease
`MacOSVersion` value. This also updates the two aforementioned areas
to use this environment variable to produce the newest stable macOS
version symbol in a more reliable way.
This extracts the logic for generating the `system_options` array in
the `replace_version_and_checksum` method into a separate
`generate_system_options` method. This logic is becoming more complex
(after recent changes) and manually testing it is a pain, so this
change is intended to allow us to add tests. The tests added here
provide 100% coverage for the method.
