Set Ruby version to 2.7.4 in Jekyll workflow

Author: vic-moreno

.github/workflows/jekyll.yml

@@ -35,6 +35,7 @@ jobs:
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1
         with:
+          ruby-version: '2.7.4'
           bundler-cache: true # runs 'bundle install' and caches installed gems automatically
           cache-version: 1 # Increment this number if you need to re-download cached gems
       - name: Setup Pages