ci: upgrade ruby for test build web page

Author: eldipa

.github/workflows/test.yml

@@ -44,7 +44,7 @@ jobs:
       - name: Setup ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "2.6"
+          ruby-version: "3.4"
       - run: make index-links-test
       - run: ./test/github-pages.sh