Use Ruby 2.7 with GitHub Actions

Author: jigarius

.github/workflows/validation.yml

@@ -9,7 +9,7 @@ jobs:
       - name: Install Ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: '2.6'
+          ruby-version: '2.7'
       - name: Install gems
         run: gem install rubocop
       - name: Lint