Replace spring-boot-starter-web to spring-boot-starter-webflux for generated apps

[artembilan]

The current state of these stream application is about a Reactive Streams as much as possible.
It is better to expose management of the generated apps via Spring WebFlux and its Netty foundation.

Premise: the current http-source is fully reactive and expects to be exposed over Netty.
Looks like the generated app comes with Tomcat, for which Spring MVC does only a simulation for Reactive Stream over Servlet container.

[sabbyanandan]

Wow, TIL the simulation bit. Thanks for the background, @artembilan. +1 from my side!

[sobychacko]

Thanks, @artembilan. We will make these changes as part of our 2021 release train.