Update rubocop.yml

.github/workflows/rubocop.yml

@@ -12,6 +12,7 @@ jobs:
       - uses: actions/checkout@v3
       - uses: ruby/setup-ruby@v1
         with:
+          version: 2.7.8
           bundler-cache: true
       - name: rubocop
         run: bundle exec rubocop --parallel