audit_min_os: only use sparkle_min_os as fallback

`Cask::Audit#audit_min_os` uses the minimum macOS version requirement
from the app bundle and newest item in the Sparkle feed but these
values are treated equally in the code. We recently came across a
cask where the app specified High Sierra (10.13) as the minimum macOS
version but the Sparkle appcast used 26.0.0 (Tahoe) as the
`minimumSystemVersion` value, so this audit incorrectly requires
`depends_on macos: ">= :tahoe"` in the cask. The app and the upstream
website both advertise the 10.13 macOS requirement, so the appcast
value needs to be fixed.

However, this highlighted a shortcoming in the audit logic, as we
probably shouldn't allow the Sparkle minimum macOS value to override
what's specified in the app. This updates the logic to only fall back
to the `sparkle_min_os` value when `bundle_min_os` isn't `nil`, so it
won't override `bundle_min_os` when the Sparkle value is higher.

Author: samford

Library/Homebrew/cask/audit.rb

@@ -774,7 +774,7 @@ def audit_min_os
       bundle_min_os = cask_bundle_min_os
       sparkle_min_os = cask_sparkle_min_os
 
-      app_min_os = [bundle_min_os, sparkle_min_os].compact.max
+      app_min_os = bundle_min_os || sparkle_min_os
       debug_messages = []
       debug_messages << "from artifact: #{bundle_min_os.to_sym}" if bundle_min_os
       debug_messages << "from upstream: #{sparkle_min_os.to_sym}" if sparkle_min_os