Introduce common SimpleUrlHandlerMapping constructors

Prior to this commit, the SimpleUrlHandlerMapping classes in Spring MVC
and Spring Webflux only had default constructors. This lead to the fact
that users often had to explicitly invoke setUrlMap() and setOrder() on
the newly instantiated SimpleUrlHandlerMapping.

In order to simplify the programmatic setup of a SimpleUrlHandlerMapping
in common scenarios, this commit introduces the following constructors.

- SimpleUrlHandlerMapping()
- SimpleUrlHandlerMapping(Map<String, ?> urlMap)
- SimpleUrlHandlerMapping(Map<String, ?> urlMap, int order)

Closes gh-23362

src/docs/asciidoc/web/webflux-websocket.adoc

@@ -53,11 +53,9 @@ Then you can map it to a URL and add a `WebSocketHandlerAdapter`, as the followi
 		public HandlerMapping handlerMapping() {
 			Map<String, WebSocketHandler> map = new HashMap<>();
 			map.put("/path", new MyWebSocketHandler());
+			int order = -1; // before annotated controllers
 
-			SimpleUrlHandlerMapping mapping = new SimpleUrlHandlerMapping();
-			mapping.setUrlMap(map);
-			mapping.setOrder(-1); // before annotated controllers
-			return mapping;
+			return new SimpleUrlHandlerMapping(map, order);
 		}
 
 		@Bean