cask/audit: reject new casks requiring Rosetta #24309

• macOS 27 is now the latest stable release so, per docs/Acceptable-Casks.md, new casks needing Rosetta 2 are no longer acceptable. Error in audit_rosetta when a new cask's artifacts are x86_64-only.


• Drop the pre-Big Sur guards: Big Sur is the oldest allowed macOS. The depends_on macos: guard also skipped every cask without one, as nil&.maximum_version.to_s is "", which sorts before "11".

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Claude with Opus 5.5 at High effort, with local review and testing.