Deprecate ListenableFuture in favor of CompletableFuture

This commit deprecates ListenableFuture in favor of CompletableFuture.
ListenableFuture was introduced in Spring Framework 4.0, when
CompletableFuture was not yet available. Spring now requires JDK 17, so
having our own type no longer seems necessary.

Major changes in this commit include:
- Deprecation of ListenableFuture and related types
  (ListenableFutureCallback, SettableListenableFuture, etc.)
- Deprecation of AsyncListenableTaskExecutor in favor of default methods
  in AsyncTaskExecutor (submitCompletable).
- AsyncHandlerMethodReturnValueHandler now has toCompletableFuture
  instead of toListenableFuture.
- WebSocketClient now has execute methods, which do the same as
  doHandshake, but return CompletableFutures (cf. the reactive
  WebSocketClient).

All other changes
- add an overloaded method that takes a CompletableFuture parameter
  instead of ListenableFuture, and/or
- add a method with a 'Async' suffix that returns a CompletableFuture
  instead of a ListenableFuture (connectAsync, sendAsync).

Closes gh-27780

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/DeferredResultReturnValueHandlerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -89,6 +89,7 @@ public class DeferredResultReturnValueHandlerTests {
 	}
 
 	@Test
+	@SuppressWarnings("deprecation")
 	public void listenableFuture() throws Exception {
 		SettableListenableFuture<String> future = new SettableListenableFuture<>();
 		testHandle(future, ListenableFuture.class, () -> future.set("foo"), "foo");
@@ -107,6 +108,7 @@ public class DeferredResultReturnValueHandlerTests {
 	}
 
 	@Test
+	@SuppressWarnings("deprecation")
 	public void listenableFutureWithError() throws Exception {
 		SettableListenableFuture<String> future = new SettableListenableFuture<>();
 		IllegalStateException ex = new IllegalStateException();