Use Ruby 3.1 for GHA builds

Author: ben

.github/workflows/pr-build.yml

@@ -13,7 +13,7 @@ jobs:
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.7
+        ruby-version: 3.1
         bundler-cache: true # runs 'bundle install' and caches installed gems automatically
 
     - name: Build book