Update jdk in workflow

Author: undernorthernsky

.github/workflows/main.yml

@@ -10,13 +10,13 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
-      - name: Set up JDK 11
+      - name: Set up JDK 21
         uses: actions/setup-java@v2
         with:
-          java-version: '11'
+          java-version: '21'
           distribution: 'adopt'
 
       - name: Grant execute permission for gradlew
         run: chmod +x gradlew
       - name: Test with Gradle
-        run: ./gradlew test
+        run: ./gradlew test