Previously, the locale context resolver used with WebFlux could only be
configured by provided a custom LocaleContextResolver bean. By
constrast, when using Spring MVC, the spring.mvc.locale and
spring.mvc.locale-resolver properties could be used to configure the
locale and the resolver (fixed or Accept header) respectively.

This commit introduces spring.web.locale and spring.web.locale-resolver
properties and deprecates their spring.mvc equivalents. The new
properties can be used to configure locale resolution with either
Spring MVC or WebFlux.

Closes gh-23449