Upgrade the samples to Spring Boot 2.4.x

Closes gh-723

samples/rest-notes-spring-hateoas/src/main/java/com/example/notes/NullOrNotBlank.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014 the original author or authors.
+ * Copyright 2014-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -35,7 +35,7 @@ import org.hibernate.validator.constraints.ConstraintComposition;
 @NotBlank
 @Target({ ElementType.FIELD, ElementType.PARAMETER })
 @Retention(RetentionPolicy.RUNTIME)
-public @interface NullOrNotBlank {
+@interface NullOrNotBlank {
 
 	String message() default "Must be null or not blank";