bump-cask-pr: add support for :latest

[SeekingMeaning]

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This is a follow-up PR to #7986

Allows for updating a cask from an old version to :latest, or from :latest to a new version

Related: Homebrew/homebrew-cask#89077

[core-code]

not sure if its interesting just wanted to mention i got this in BigSur

Error: cannot load such file -- ruby-progressbar
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
/usr/local/Homebrew/Library/Homebrew/vendor/bundle/ruby/2.6.0/gems/rubocop-0.90.0/lib/rubocop/formatter/fuubar_style_formatter.rb:3:in `<top (required)>'
/usr/local/Homebrew/Library/Homebrew/vendor/bundle/ruby/2.6.0/gems/rubocop-0.90.0/lib/rubocop.rb:590:in `require_relative'
/usr/local/Homebrew/Library/Homebrew/vendor/bundle/ruby/2.6.0/gems/rubocop-0.90.0/lib/rubocop.rb:590:in `<top (required)>'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
/usr/local/Homebrew/Library/Homebrew/style.rb:76:in `run_rubocop'
/usr/local/Homebrew/Library/Homebrew/style.rb:52:in `check_style_impl'
/usr/local/Homebrew/Library/Homebrew/style.rb:15:in `check_style_and_print'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/style.rb:24:in `run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/abstract_command.rb:89:in `run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd.rb:237:in `run'
/usr/local/Homebrew/Library/Homebrew/cask/cmd.rb:165:in `run'
/usr/local/Homebrew/Library/Homebrew/cmd/cask.rb:14:in `cask'
/usr/local/Homebrew/Library/Homebrew/brew.rb:119:in `<main>'
Error: `brew cask style --fix` failed!

[SeekingMeaning]

Doesn't seem to related to this pull request (Could you open an issue for this?)

[issyl0]

Big Sur beta is officially unsupported, so diagnosing and fixing the issue in a PR is the way to go please @core-code.