Align simple sample with configuration properties validation change

See gh-8024

spring-boot-samples/spring-boot-sample-simple/src/main/java/sample/simple/SampleConfigurationProperties.java

@@ -20,7 +20,9 @@ import javax.validation.constraints.NotNull;
 
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.stereotype.Component;
+import org.springframework.validation.annotation.Validated;
 
+@Validated
 @Component
 @ConfigurationProperties(prefix = "sample")
 public class SampleConfigurationProperties {