Use JDK 21 for CI

Author: erwin-kok

.github/workflows/ci.yaml

@@ -6,11 +6,11 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
-      - name: Set up JDK 17
+      - name: Set up JDK 21
         uses: actions/setup-java@v3
         with:
           distribution: 'temurin'
-          java-version: 17
+          java-version: 21
           cache: 'gradle'
 
       - name: Gradle build