Configuration options for virtual threads (on JDK 21)

VirtualThreadDelegate built on JDK 21 for multi-release jar.
Includes dedicated VirtualThreadTaskExecutor as lean option.
Includes setVirtualThreads flag on SimpleAsyncTaskExecutor.
Includes additional default methods on AsyncTaskExecutor.

Closes gh-30241

spring-context-support/src/main/java/org/springframework/scheduling/quartz/SimpleThreadPoolTaskExecutor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -77,12 +77,6 @@ public class SimpleThreadPoolTaskExecutor extends SimpleThreadPool
 		}
 	}
 
-	@Deprecated
-	@Override
-	public void execute(Runnable task, long startTimeout) {
-		execute(task);
-	}
-
 	@Override
 	public Future<?> submit(Runnable task) {
 		FutureTask<Object> future = new FutureTask<>(task, null);