update to Graal 21 for CI

Author: danthe1st

.github/workflows/build.yml

@@ -27,10 +27,10 @@ jobs:
     if: ${{ (github.event_name == 'push' && github.ref == 'refs/heads/main') || github.event_name == 'workflow_dispatch' }}
     steps:
       - uses: actions/checkout@v4
-      - name: Set up JDK 17
+      - name: Set up JDK 21
         uses: graalvm/setup-graalvm@v1
         with:
-          java-version: '17'
+          java-version: '21'
           distribution: 'graalvm-community'
       - name: Build JAR
         run: ./gradlew nativeCompile