My previous plan to enable frozen-string-literal across all Rails gems didn't work (#29540) and had to be reverted because not all components were ready for it. Let's take a more incremental approach of enabling it per gem with the rubocop Only option, only for gems that we know are ready for frozen string.

Style/FrozenStringLiteralComment:
  Enabled: true
  EnforcedStyle: always
  Include:
    - 'activesupport/**/*'

Thanks to @pat 's work, activesupport is now ready for frozen string literal.

Once #29655 is merged, I'll do the same for the rest of components.

@rafaelfranca @matthewd