deprecate_disable: add cask deprecation reason

[bevanjkay]

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Adds an additional deprecate!/disable! reason for casks, for when an app moves to the Mac App Store.
Would be used for Homebrew/homebrew-cask#170096 and other similar circumstances.

[apainintheneck]

How often does this happen?

[bevanjkay]

I'm not sure exactly, but I would say common enough (5+ a year) to warrant it being a listed reason.

[MikeMcQuaid]

Thanks again @bevanjkay!