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-context/src/main/java/org/springframework/context/event/ApplicationListenerMethodAdapter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 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.
@@ -262,6 +262,7 @@ public class ApplicationListenerMethodAdapter implements GenericApplicationListe
 		return new Object[] {event};
 	}
 
+	@SuppressWarnings("deprecation")
 	protected void handleResult(Object result) {
 		if (reactiveStreamsPresent && new ReactiveResultHandler().subscribeToPublisher(result)) {
 			if (logger.isTraceEnabled()) {

spring-context/src/main/java/org/springframework/scheduling/annotation/AsyncResult.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 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.
@@ -45,7 +45,9 @@ import org.springframework.util.concurrent.SuccessCallback;
  * @see Async
  * @see #forValue(Object)
  * @see #forExecutionException(Throwable)
+ * @deprecated as of 6.0, in favor of {@link CompletableFuture}
  */
+@Deprecated
 public class AsyncResult<V> implements ListenableFuture<V> {
 
 	@Nullable

spring-context/src/main/java/org/springframework/scheduling/concurrent/ConcurrentTaskExecutor.java

@@ -62,6 +62,7 @@ import org.springframework.util.concurrent.ListenableFuture;
  * @see DefaultManagedTaskExecutor
  * @see ThreadPoolTaskExecutor
  */
+@SuppressWarnings("deprecation")
 public class ConcurrentTaskExecutor implements AsyncListenableTaskExecutor, SchedulingTaskExecutor {
 
 	@Nullable

spring-context/src/main/java/org/springframework/scheduling/concurrent/ThreadPoolTaskExecutor.java

@@ -80,7 +80,7 @@ import org.springframework.util.concurrent.ListenableFutureTask;
  * @see ThreadPoolExecutorFactoryBean
  * @see ConcurrentTaskExecutor
  */
-@SuppressWarnings("serial")
+@SuppressWarnings({"serial", "deprecation"})
 public class ThreadPoolTaskExecutor extends ExecutorConfigurationSupport
 		implements AsyncListenableTaskExecutor, SchedulingTaskExecutor {
 

spring-context/src/main/java/org/springframework/scheduling/concurrent/ThreadPoolTaskScheduler.java

@@ -59,7 +59,7 @@ import org.springframework.util.concurrent.ListenableFutureTask;
  * @see #setThreadFactory
  * @see #setErrorHandler
  */
-@SuppressWarnings("serial")
+@SuppressWarnings({"serial", "deprecation"})
 public class ThreadPoolTaskScheduler extends ExecutorConfigurationSupport
 		implements AsyncListenableTaskExecutor, SchedulingTaskExecutor, TaskScheduler {