Gradle template to use Java 21

jme3-templates/src/com/jme3/gde/templates/files/freemarker/build.gradle.ftl

@@ -40,7 +40,7 @@ project(":assets") {
 
     java {
         toolchain {
-            languageVersion = JavaLanguageVersion.of(17)
+            languageVersion = JavaLanguageVersion.of(21)
         }
     }
 }
@@ -114,7 +114,7 @@ jar {
 
 java {
     toolchain {
-        languageVersion = JavaLanguageVersion.of(17)
+        languageVersion = JavaLanguageVersion.of(21)
     }
 }