Bump ruby/setup-ruby from 1.207.0 to 1.208.0 (#597)

Bumps [ruby/setup-ruby](https://github.com/ruby/setup-ruby) from 1.207.0 to 1.208.0.
- [Release notes](https://github.com/ruby/setup-ruby/releases)
- [Changelog](https://github.com/ruby/setup-ruby/blob/master/release.rb)
- [Commits](ruby/setup-ruby@4a9ddd6...868b3f0)

---
updated-dependencies:
- dependency-name: ruby/setup-ruby
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/gh-pages.yml

@@ -28,7 +28,7 @@ jobs:
         with:
           fetch-depth: 0 # need full history to get page last modified times
       - name: Setup Ruby
-        uses: ruby/setup-ruby@4a9ddd6f338a97768b8006bf671dfbad383215f4 # v1.207.0
+        uses: ruby/setup-ruby@868b3f088412f139260f27f5b148179b9dd6b008 # v1.208.0
         with:
           ruby-version: '3.1'
           bundler-cache: true # runs 'bundle install' and caches installed gems automatically