Stop calling `brew tap homebrew/cask`

What does this merge request do and why?

Per https://github.com/Homebrew/brew/pull/15633, calling brew tap homebrew/cask is now deprecated and no longer needed:

Warning: Tapping homebrew/cask is no longer typically necessary.
Add --force if you are sure you need it done.

brew install --cask <package> can function without calling brew tap homebrew/cask.

This also saves 414 MB:

% brew untap homebrew/cask
Untapping homebrew/cask...
Untapped 4290 casks (4,708 files, 413.9MB).

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.