SpringBoot Add ThreadPoolTaskExecutorBuilder as a replacement for TaskExecutorBuilder

ThreadPoolTaskExecutorBuilder will be a replacement for the to-deprecate TaskExecutorBuilder.

With #35711 we think about introducing a builder for SimpleAsyncTaskExecutor, which can handle virtual threads.

Comment From: mhalbritter

The bean arrangement works this way:

If users don't have a custom TaskExecutorBuilder: Both a TaskExecutorBuilder and a ThreadPoolTaskExecutorBuilder is available. The ThreadPoolTaskExecutorBuilder applies the TaskExecutorCustomizers, too. The ThreadPoolTaskExecutorBuilder is used to build the ThreadPoolTaskExecutor.

If users have a custom TaskExecutorBuilder: There's no ThreadPoolTaskExecutorBuilder in the context, and the TaskExecutorBuilder is used to build the ThreadPoolTaskExecutor.