Consistent abstract declaration of "Abstract" base classes

Closes gh-25240

spring-context/src/main/java/org/springframework/jmx/export/metadata/AbstractJmxAttribute.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2007 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -22,7 +22,7 @@ package org.springframework.jmx.export.metadata;
  * @author Rob Harrop
  * @since 1.2
  */
-public class AbstractJmxAttribute {
+public abstract class AbstractJmxAttribute {
 
 	private String description = "";