This commit configures the virtual threads support in Reactor Core for
Schedulers if:
* the current JDK is 21 or higher
* the current environment enables this globally with the
`"spring.threads.virtual.enabled"` property.
This needs to happen early in the application startup process, as this
feature is detected statically when the first schedulers call is made.
As a result, this is being done with an environment post processor and
not with an auto-configuration class.
Closes gh-36302
d46a58f0f6