Update annotation in MainController from @controller to @RestController

The update from @controller to @RestController in MainController.java allows automatic serialization of response objects into JSON. This simplifies the controller's implementation by obviating manual serialization handling.

messaging/producer/src/main/java/nawaphon/microservices/event_sourcing/producer/controllers/MainController.java

@@ -9,14 +9,12 @@
 import org.slf4j.LoggerFactory;
 import org.springframework.http.HttpStatus;
 import org.springframework.kafka.core.KafkaTemplate;
-import org.springframework.stereotype.Controller;
 import org.springframework.web.bind.annotation.*;
 
 import java.util.UUID;
 
-@Controller
 @RequestMapping("/")
-@ResponseBody
+@RestController
 public class MainController {
 
     private static final Logger logger = LoggerFactory.getLogger(MainController.class);