Explicitly set asyncSupported for reactive servers

Update `JettyReactiveWebServerFactory`/`TomcatReactiveWebServerFactory`
to explicitly set `asyncSupported` to `true`.

Fixes gh-12486

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

@@ -140,6 +140,7 @@ public class JettyReactiveWebServerFactory extends AbstractReactiveWebServerFact
 		Server server = new Server(getThreadPool());
 		server.addConnector(createConnector(address, server));
 		ServletHolder servletHolder = new ServletHolder(servlet);
+		servletHolder.setAsyncSupported(true);
 		ServletContextHandler contextHandler = new ServletContextHandler(server, "",
 				false, false);
 		contextHandler.addServlet(servletHolder, "/");

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatReactiveWebServerFactory.java

@@ -131,7 +131,7 @@ public class TomcatReactiveWebServerFactory extends AbstractReactiveWebServerFac
 		loader.setLoaderClass(TomcatEmbeddedWebappClassLoader.class.getName());
 		loader.setDelegate(true);
 		context.setLoader(loader);
-		Tomcat.addServlet(context, "httpHandlerServlet", servlet);
+		Tomcat.addServlet(context, "httpHandlerServlet", servlet).setAsyncSupported(true);
 		context.addServletMappingDecoded("/", "httpHandlerServlet");
 		host.addChild(context);
 		configureContext(context);