Build with Java 21

Author: nakamura-to

.github/workflows/ci.yml

@@ -14,11 +14,11 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up JDK 11
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
-          java-version: 11
+          java-version: 21
 
       - name: Grant execute permission for gradlew
         run: chmod +x gradlew