Provide properties to configure WebFlux's locale and how it's resolved

[wilkinsona]

MVC has locale and locale-resolver properties that control the locale and how it's resolved (fixed or via the Accept-Language header). We should offer the same with WebFlux.

[wilkinsona]

This would give us the following properties:

• spring.mvc.locale
• spring.mvc.locale-resolver
• spring.webflux.locale
• spring.webflux.locale-resolver

The properties would behave the same with both WebFlux and MVC. We could, perhaps, consider common properties that work with both web stacks. That would align with, for example, spring.resources.* that both MVC and WebFlux consume.

[philwebb]

We're going to try:

spring.web.locale-resolver=fixed
spring.web.locale=en_GB

We'll also consider moving spring.resources.* under it.

[wilkinsona]

I've opened #23917 for the spring.resources.* move.

[wilkinsona]

This issue is noteworthy due to the deprecation of the spring.mvc.locale and spring.mvc.locale-resolver properties.