Update ruby version in CI

Author: mullermp

.github/workflows/test-on-push-and-pr.yml

@@ -15,6 +15,6 @@ jobs:
     - name: Set up ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.7
+        ruby-version: 3.1
     - name: Run 'pr' target
       run: make pr