Update dependency ruby to v2.7.8

Author: renovate[bot]

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
       - name: Setup ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: '2.7'
+          ruby-version: '2.7.8'
           bundler-cache: true
       - name: Run rake validate
         run: bundle exec rake validate