Add @since in ApplicationContextInitializedEvent

Closes gh-14579

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/event/ApplicationContextInitializedEvent.java

@@ -26,6 +26,7 @@ import org.springframework.context.ConfigurableApplicationContext;
  * called but before any bean definitions are loaded.
  *
  * @author Artsiom Yudovin
+ * @since 2.1.0
  */
 @SuppressWarnings("serial")
 public class ApplicationContextInitializedEvent extends SpringApplicationEvent {