wip use ruby 3.1 for Rails 8 build

Author: toshimaru

.github/workflows/ci.yml

@@ -69,7 +69,7 @@ jobs:
             refs/tags/v5.2*:refs/tags/v5.2*
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 2.7
+          ruby-version: 3.1
           bundler-cache: true
       - name: Doc Build
         run: rake build_multi