Remove remaining references to no longer supported container versions

Closes gh-11504

spring-boot-project/spring-boot-docs/src/main/asciidoc/getting-started.adoc

@@ -57,7 +57,7 @@ Spring Boot supports the following embedded servlet containers:
 |Jetty 9.4
 |3.1
 
-|Undertow 1.3
+|Undertow 1.4
 |3.1
 |===
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/jetty/JettyServletWebServerFactory.java

@@ -273,8 +273,6 @@ public class JettyServletWebServerFactory extends AbstractServletWebServerFactor
 					root == null ? rootResource : new LoaderHidingResource(rootResource));
 			for (URL resourceJarUrl : this.getUrlsOfJarsWithMetaInfResources()) {
 				Resource resource = createResource(resourceJarUrl);
-				// Jetty 9.2 and earlier do not support nested jars. See
-				// https://github.com/eclipse/jetty.project/issues/518
 				if (resource.exists() && resource.isDirectory()) {
 					resources.add(resource);
 				}