Revisit WebFlux.fn auto-configuration after SPR-15536

[bclozel]

With SPR-15536, Spring Framework is considering bringing WebFlux.fn's configuration under the @EnableWebFlux umbrella.

This would simplify the auto-configuration arrangement and allow to use both annotation and functional variants in the same application. Many of the downsides listed in SPR-15405 would be fixed — mapping several handlers with the same pattern would still remain as an issue, but it's in the control of the developer.