Migrate to `Homebrew/homebrew-cask`

[ldennington]

GCM is currently published to a private tap. However, in an effort to standardize macOS installs, we plan to migrate it to the official Homebrew tap for casks (Homebrew/homebrew-cask).

The following are the requirements we believe will need to be fulfilled for this effort:

• Adhere to the steps outlined in the Cask Cookbook
• Ensure the new cask works for both x64 and arm64 runtimes
• Merge PR with new cask into Homebrew/homebrew-cask.
• Update GCM versioning
• Update mjcheetham/update-homebrew to update new cask
• Remove private tap from GCM Homebrew instructions.
• Add a tap_migrations.json to microsoft/homebrew-git to ensure users are re-directed to the new tap.
• Update releases to publish to Homebrew/homebrew-cask instead of the private tap.

[ldennington]

Completed with:

• Add git-credential-manager v2.3.0 Homebrew/homebrew-cask#152552
• homebrew-cask: update according to new sharding mjcheetham/update-homebrew#19
• release: update Homebrew tap #1374
• gcm: remove cask microsoft/homebrew-git#73
• docs: remove GCM from list of casks microsoft/homebrew-git#74