As of Spring Framework 5.1, we're depending on the Reactor Californium
release train.
Reactor Netty is now at version 0.8 and changed its artifact
coordinates, package names and broke several APIs. Spring Framework is
now up-to-date with those changes and this commit does the same for
Spring Boot.

Note that in that process, the `NettyServerCustomizer` has been changed
since the former `HttpServerOptions.Builder` API is now gone from
Reactor Netty, and we're now relying on immutable server instances
instead of a stateful builder pattern.

See gh-13321