Add since tag

See gh-19647

spring-test/src/main/java/org/springframework/test/web/reactive/server/CookieAssertions.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 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.
@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.test.web.reactive.server;
 
 import java.time.Duration;
@@ -28,7 +29,9 @@
 
 /**
  * Assertions on cookies of the response.
+ *
  * @author Rossen Stoyanchev
+ * @since 5.3
  */
 public class CookieAssertions {