@ModelAttribute name attribute is not supported in WebFlux

[sbrannen]

While reviewing #28422, I noticed that the name attribute in @ModelAttribute is not supported in WebFlux.

This is because MethodParameter is used instead of SynthesizingMethodParameter when retrieving the @ModelAttribute annotation. In other words, @AliasFor is not honored because the annotation is not synthesized. Consequently, only the value attribute is supported in WebFlux when specifying a custom name via @ModelAttribute.