fix JS default ot ES6 (OpenAPITools#2084)

Author: wing328

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/JavascriptClientCodegen.java

@@ -215,7 +215,7 @@ public void processOpts() {
         if (additionalProperties.containsKey(USE_ES6)) {
             setUseES6(convertPropertyToBooleanAndWriteBack(USE_ES6));
         } else {
-            setUseES6(false); // default to ES5
+            setUseES6(true); // default to ES6
         }
         super.processOpts();