Update JDK version in main.yml

Author: hanruwang

.github/workflows/main.yml

@@ -24,6 +24,11 @@ jobs:
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - uses: actions/checkout@v2
+      # Use a non-default JDK version
+      - uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: '21'
 
       # Runs a single command using the runners shell
       - name: Perform a Gradle build