rubocop: Set TargetRubyVersion to 3.1 & disable all cops that fail

- As discussed in
  https://github.com/Homebrew/brew/pull/16337#issuecomment-1855668516,
  then we won't have the chicken/egg problem of fixing cops for syntax
  that's unsupported on Ruby 2.6 _before_ we set the TargetRubyVersion
  to 3.1, but if we set it too early then all the taps will fail.