Remove support for Mavericks

[claui]

With Homebrew/brew#7698, Homebrew no longer runs on Mavericks.

In preparation, Homebrew/brew#7716 has been merged, which removes support for the :mavericks symbol.

This causes brew cask install and brew cask uninstall to fail with the following error message if cask-versions is tapped:

Error: Cask 'brave-browser-nightly' definition is invalid: invalid 'depends_on macos' value: ">= :mavericks"

This PR fixes the issue by removing all references to :mavericks.

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After making all changes to a cask, verify:

• brew cask audit --download {{cask_file}} is error-free. Fails for one of the casks (403 error for brave-browser-dev) but that’s unrelated to this PR.
• brew cask style --fix {{cask_file}} reports no offenses.
• There are no open pull requests for the same update.
• The submission is for a stable version or documented exception.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• brew cask install {{cask_file}} worked successfully.
• brew cask uninstall {{cask_file}} worked successfully.
• Checked the cask was not already refused.
• Checked the cask is submitted to the correct repo.