Use Ruby 3.1 for jekyll-build

Author: toshimaru

.github/workflows/ci.yml

@@ -1,5 +1,7 @@
 name: CI
+
 on: [pull_request]
+
 jobs:
   jekyll-build:
     runs-on: ubuntu-latest
@@ -8,10 +10,11 @@ jobs:
     - uses: actions/checkout@v2
     - 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
+
   doc-build:
     runs-on: ubuntu-latest
     name: Rails Doc Build