Prior to this commit, Spring Boot would use `Schedulers.elastic()` when
required to process blocking tasks in a reactive environment.
reactor/reactor-core#1804 introduced a new scheduler,
`Schedulers.boundedElastic()` that behaves quite similarly but:

* will limit the number of workers thread
* will queue tasks if no worker thread is available and reject them is
the queue is exceeds a limit

This allows Spring Boot to schedule blocking tasks as before and allows
greater flexibility.

Fixes gh-18269
See gh-18276