Rename EmbeddedServletContainer -> EmbeddedWebServer

This contract is not specific to servlet containers and should be
reused by all web server implementations (including reactive variants).

Fixes gh-8208

spring-boot-samples/spring-boot-sample-tomcat/src/test/java/sample/tomcat/SampleTomcatApplicationTests.java

@@ -90,7 +90,7 @@ public class SampleTomcatApplicationTests {
 	public void testTimeout() throws Exception {
 		EmbeddedWebApplicationContext context = (EmbeddedWebApplicationContext) this.applicationContext;
 		TomcatEmbeddedServletContainer embeddedServletContainer = (TomcatEmbeddedServletContainer) context
-				.getEmbeddedServletContainer();
+				.getEmbeddedWebServer();
 		ProtocolHandler protocolHandler = embeddedServletContainer.getTomcat()
 				.getConnector().getProtocolHandler();
 		int timeout = ((AbstractProtocol<?>) protocolHandler).getConnectionTimeout();