Use Reactor's new Schedulers.boundedElastic() #18269
Description
As of reactor/reactor-core#1804, Reactor core will provide a new Scheduler implementation, Schedulers.boundedElastic().
This implementation, unlike elastic does not create an unbounded number of threads; it creates a bounded number of workers and will queue tasks if there are no worker available. This implementation is a direct replacement for the elastic one, as it limits the amount of resources.
Spring Boot Actuator is currently using the elastic scheduler un various places (health indicators and the WebFlux EndpointHandlerMapping). We're doing so because we need to execute tasks that might be blocking and we need to ensure that we're not blocking a server thread while doing so.