Use Ruby 3.1 for latest railsdoc build

Author: toshimaru

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     - name: Jekyll Build
       run: bundle exec jekyll build
 
-  doc-build:
+  doc-build-latest:
     runs-on: ubuntu-latest
     name: Rails Doc Build
     steps:
@@ -24,7 +24,7 @@ jobs:
         submodules: true
     - uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.7
+        ruby-version: 3.1
         bundler-cache: true
     - name: Doc Build
       run: |