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-scripting/src/main/java/org/springframework/integration/scripting/AbstractScriptExecutingMessageProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -31,6 +31,8 @@ import org.springframework.util.Assert;
 /**
  * Base {@link MessageProcessor} for scripting implementations to extend.
  *
+ * @param <T> the paylaod type.
+ *
  * @author Mark Fisher
  * @author Stefan Reuter
  * @author Artem Bilan
@@ -79,7 +81,7 @@ public abstract class AbstractScriptExecutingMessageProcessor<T>
 	}
 
 	/**
-	 * Executes the script and returns the result.
+	 * Execute the script and return the result.
 	 */
 	@Override
 	@Nullable