figma-beta: migrate to `homebrew/cask` and rename to `figma@beta`

Important: Do not tick a checkbox if you haven’t performed its action. Honesty is indispensable for a smooth review process.

In the following questions <cask> is the token of the cask you're submitting.

After making any changes to a cask, existing or new, verify:

Additionally, if adding a new cask:

[ ] Named the cask according to the token reference.
[ ] Checked the cask was not already refused.
[ ] Checked the cask is submitted to the correct repo.
[ ] brew audit --cask --new <cask> worked successfully.
[ ] HOMEBREW_NO_INSTALL_FROM_API=1 brew install --cask <cask> worked successfully.
[ ] brew uninstall --cask <cask> worked successfully.

Simpler one as no conflicts to modify

Relatively lower installs but at least reasonable enough to keep around (From March 13 analytics, 85 in last year and rank 3531).

Homebrew / homebrew-cask-versions