Use the latest stable Ruby to run rubocop

Setting `ruby-version: ruby` points to the latest stable version. This removes the need to update the version each time a new Ruby release comes out.

Author: willnet

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: '3.4'
+        ruby-version: ruby
         bundler-cache: true
     - name: Run RuboCop
       run: bundle exec rubocop