This adds a `#highest_allowed` method to `MacOSRequirement`, so we can
easily identify the highest supported macOS version that a requirement
allows, if any.
This can be used when producing a minimal set of supported macOS
versions that meet requirements in `UsesOnSystem.macos_requirements`.
One of the intended use cases for this is to identify which macOS
versions we could simulate to work with all variations of a cask that
uses macOS on_system blocks.