Explicit 3.1 Ruby in test suite

Author: kddnewton

.github/workflows/main.yml

@@ -13,7 +13,7 @@ jobs:
     - uses: ruby/setup-ruby@v1
       with:
         bundler-cache: true
-        ruby-version: 3.0
+        ruby-version: '3.1'
     - name: Test
       run: bundle exec rake test
   automerge: