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-cli/samples/http.groovy

@@ -16,7 +16,7 @@ class Example implements CommandLineRunner {
 	}
 
 	void run(String... args) {
-		def port = context.embeddedServletContainer.port;
+		def port = context.embeddedWebServer.port;
 		def world = new RESTClient("http://localhost:" + port).get(path:"/").data.text
 		print "Hello " + world
 	}