Configure suitable TaskExecutor for WebSocket when virtual threads are enabled

[snicoll]

A recent issue in Spring Framework (spring-projects/spring-framework#32081) made us aware that we overlooked websocket broker support for virtual threads.

Now that ChannelRegistration allows a plain TaskExecutor to be specified via the new executor method, it would be nice to configure it when virtual threads are enabled.

[BenchmarkingBuffalo]

HI @wilkinsona,
I can work on this, once spring 6.1.4 is out.
You can assign it to me, if you like.

[mhalbritter]

Superseded by #39611.