Use Ruby 3.1 for gh-pages deploy workflow

Author: toshimaru

.github/workflows/gh-pages.yml

@@ -11,7 +11,7 @@ jobs:
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 3.0
+        ruby-version: 3.1
         bundler-cache: true 
     - name: Jekyll Build
       run: bundle exec jekyll build