Build with java 21 as default environment

Signed-off-by: cpw <[email protected]>

.github/workflows/gradle.yml

@@ -9,10 +9,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v1
-    - name: Set up JDK 1.17
+    - name: Set up JDK 1.21
       uses: actions/setup-java@v1
       with:
-        java-version: 1.17
+        java-version: 1.21
     - name: Build with Gradle
       run: ./gradlew build
     - name: Curseforge publish