moderndeck: deprecate

[samford]

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:

• The submission is for a stable version or documented exception.
• brew audit --cask --online <cask> is error-free.
• brew style --fix <cask> reports no offenses.

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.
• brew install --cask <cask> worked successfully.
• brew uninstall --cask <cask> worked successfully.

---

The GitHub repository for moderndeck was archived on 2023-07-04. The homepage redirects to an issue explaining that Twitter API changes made ModernDeck unusable and that development on the project was halted. The README was updated to the link to the same issue before the repository was archived, making it clear that this isn't being developed anymore.

This PR deprecates the moderndeck cask accordingly and removes the livecheck block (so it will be automatically skipped as deprecated).

[miccal]

Thank you as always @samford.