Make use of new GetMapping and PostMapping annotations

Closes gh-5277

spring-boot-samples/spring-boot-sample-web-mustache/src/main/java/sample/mustache/WelcomeController.java

@@ -22,6 +22,7 @@ import java.util.Map;
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.http.HttpStatus;
 import org.springframework.stereotype.Controller;
+import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.ResponseStatus;
 
@@ -31,7 +32,7 @@ public class WelcomeController {
 	@Value("${application.message:Hello World}")
 	private String message = "Hello World";
 
-	@RequestMapping("/")
+	@GetMapping("/")
 	public String welcome(Map<String, Object> model) {
 		model.put("time", new Date());
 		model.put("message", this.message);