Tell users to delete old caskroom if it exists

[jawshooah]

Changes to the core

• Followed hacking.md.

Refs #22359

[adidalal]

LGTM

[joewiz]

As the OP states in #21913 says, it would be great to give crystal clear, fool proof directions for migrating to the new location. The discussion in that issue gave some ideas, but even the one that seemed to garner the most praise contained a syntax error. I've summarized the approach and corrected the code in the quoted portion below. Would you consider adding this?

To move your Casks to the new, recommended location, use this command:
mv /opt/homebrew-cask/Caskroom /usr/local
Please note that any casks installed prior to #13966 were symlinked rather than moved to their destinations, so you'll need to re-install those manually. To simply reinstall all Casks, use this command:
for cask in $(brew cask list); do brew cask install $cask --force; done;

[vitorgalvao]

To simply reinstall all Casks, use this command:
for cask in $(brew cask list); do brew cask install $cask --force; done;

As has been said repeatedly, brew cask list is broken, you cannot rely on it. There’s no simple foolproof command to this migration. Best we can do is give guidelines, which is what we already do.

[joewiz]

Ah, I didn't realize that. Thanks for the reference to #14058.