Enable virtual thread support in Jetty12

I was reading https://jetty.org/docs/jetty/12/programming-guide/arch/threads.html#thread-pool-virtual-threads, and wondered if it would make sense to make this configurable in Felix HTTP bundle for jetty12. 

It would need adding here (conditionally), probably via using reflection to support pre jdk 21. 

https://github.com/apache/felix-dev/blob/8b2d2faa3669472f50020f7165a076af103c76e9/http/jetty12/src/main/java/org/apache/felix/http/jetty/internal/JettyService.java#L279

cziegeler what do you think?