Document that graceful shutdown requires Tomcat 9.0.33

Closes gh-20886

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-features.adoc

@@ -3151,6 +3151,8 @@ The exact way in which new requests are not permitted varies depending on the we
 Jetty, Reactor Netty, and Tomcat will stop accepting requests at the network layer.
 Undertow will accept requests but respond immediately with a service unavailable (503) response.
 
+NOTE: Graceful shutdown with Tomcat requires Tomcat 9.0.33 or later.
+
 Graceful shutdown occurs as one of the first steps during application close processing and before any beans have been destroyed.
 This ensures that the beans are available for use by any processing that occurs while in-flight requests are being allowed to complete.
 To enable graceful shutdown, configure the configprop:server.shutdown.grace-period[] property, as shown in the following example: