Suggestion: new 'github-pages' value for 'ruby-version'

[maxim-belkin]

It would've been great if we could say:

      - uses: actions/setup-ruby@v1
        with:
          ruby-version: 'github-pages'

to use Ruby that is currently required by the github-pages gem.
This version can be extracted from https://pages.github.com/versions/ or https://pages.github.com/versions.json.