SpringBoot Use virtual threads in BackgroundPreinitializer if enabled

Comment From: mhalbritter

I've reverted the changes. We'll run some benchmarks if virtual threads have an impact on startup performance before putting it back in.

Comment From: mhalbritter

Improved changes are here: https://github.com/mhalbritter/spring-boot/tree/mh/36695-use-virtual-threads-in-backgroundpreinitializer-if-enabled

Comment From: mhalbritter

We run some benchmarks, and the virtual threads don't change anything to the better here.