Checkstyle changes

* Upgrade to Checkstyle 9.0
* apply some JavaDocs rules
* Fix JavaDocs rules violations
* Some other minor clean up in the affected classes

spring-integration-stomp/src/main/java/org/springframework/integration/stomp/support/IntegrationStompHeaders.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015-2019 the original author or authors.
+ * Copyright 2015-2021 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.
@@ -26,8 +26,10 @@ import org.springframework.messaging.simp.stomp.StompHeaders;
  * The STOMP headers with Integration-friendly {@code stomp_} prefix.
  *
  * @author Artem Bilan
- * @see StompHeaders
+ *
  * @since 4.2
+ *
+ * @see StompHeaders
  */
 public abstract class IntegrationStompHeaders {