Set jdk in pages workflow explicitly

Author: nsk90

.github/workflows/pages.yml

@@ -36,6 +36,12 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+      - name: Set up JDK
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: '21'
+
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1
         with: