cask/audit: support on_os blocks in audit_min_os

[EricFromCanada]

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As suggested in Homebrew/homebrew-cask#176710, this PR does the following:

• to MacOSRequirement, adds and tests minimum_version to return the minimum OS version allowed by a depends_on macos: stanza
• to OnSystem, adds @on_system_block_min_os which is made available via cask/dsl.rb for fetching the minimum OS version of the current system-specific block
• in Cask::Audit#audit_min_os:
  • checks the detected minimum supported OS version of the artifact against the minimum OS version of either the current system-specific block if present or the depends_on stanza (and adjust the associated error message to match)
  • fixes a potential issue when checking a cask specifying a depends_on array of allowed OS versions by calling minimum_version instead of version

This should help us notice when a cask version bump introduces a new minimum OS requirement.

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[Rylan12]

Makes sense to me!