Refine `RouterFunctionMapping` ordering consistency

[sdeleuze]

Related to spring-projects/spring-boot#34846, it would be great to refine if possible the relative ordering between RouterFunctionMapping and other mapping to have a better consistency between Spring WebFlux (where RouterFunctionMapping is before RequestMappingHandlerMapping) and Spring MVC (where this is the other way around).

[sdeleuze]

We are going to change, as of Spring Framework 6.1, the order of RouterFunctionMapping defined in WebMvcConfigurationSupport from 3 to -1 in order to achieve better consistency and reduce the surprise factor. That way, functional routes will always be before annotation-based ones.

It also makes sense from a user perspective since functional routes are kind of lower level than annotation-based ones, and the overhead will be minimal for pure annotation-based use cases because if there are no RouterFunction in the application context, then RouterFunctionMapping::routerFunction is null, and the mapping does not allocate anything (no Optional<HandlerFunction<T>> instance have to be created).