fix(kotlin-client): remove upstream guard blocking Jackson 3 support

The upstream kotlin-spring PR added a guard throwing IllegalArgumentException
for useJackson3 on kotlin-client. Since this branch implements that support,
remove the guard while keeping the proper validation below it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: yonatankarp

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

@@ -469,11 +469,6 @@ public void processOpts() {
             convertPropertyToBooleanAndWriteBack(USE_SPRING_BOOT3);
         }
 
-        if (isUseJackson3()) {
-            throw new IllegalArgumentException(
-                "useJackson3 is not yet supported for kotlin-client. Jackson 3 support for kotlin-client will be added in a future release.");
-        }
-
         if (additionalProperties.containsKey(CodegenConstants.SERIALIZATION_LIBRARY)) {
             setSerializationLibrary((String) additionalProperties.get(CodegenConstants.SERIALIZATION_LIBRARY));
             additionalProperties.put(this.serializationLibrary.name(), true);