build: use java 21 in github actions

.github/workflows/jvm-ci.yml

@@ -21,6 +21,11 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
+      - uses: actions/setup-java@v4
+        with:
+          distribution: "adopt-openj9"
+          java-version: "21"
+
       - run: for dir in */; do
           cd $dir;
           ./gradlew test;