Provide first-class virtual thread option on `ThreadPoolTaskExecutor`/`ThreadPoolTaskScheduler`

[jhoeller]

As mentioned in ##33780 (comment), there are scenarios where a ThreadPoolTaskExecutor or ThreadPoolTaskScheduler benefits from being configured with a factory for virtual threads. In contrast to SimpleAsyncTaskExecutor/SimpleAsyncTaskScheduler, this is naturally integrated with Spring's lifecycle management for stopping and restarting execution threads, including an early stop signal for a graceful shutdown arrangement. For 6.2, we can provide first-class virtual thread support through a setVirtualThreads(true) flag on ThreadPoolTaskExecutor and ThreadPoolTaskScheduler.