Switches appveyor CI to JDK21

Author: damianszczepanik

appveyor.yml

@@ -13,7 +13,7 @@ environment:
     # https://stackoverflow.com/questions/42024619/maven-build-gets-connection-reset-when-downloading-artifacts
     MAVEN_OPTS: "-Dhttp.keepAlive=false -Dmaven.wagon.http.retryHandler.count=3"
   matrix:
-    - JAVA_HOME: C:\Program Files\Java\jdk11
+    - JAVA_HOME: C:\Program Files\Java\JDK21
 
 build_script:
   - mvn clean package --batch-mode -DskipTest