Rework spring.mustache properties to make it clear which are Servlet-specific

[wilkinsona]

Split out from #28858.

The following properties are Servlet-specific but that's not apparent from their prefix:

spring.mustache.allow-request-override
spring.mustache.allow-session-override
spring.mustache.cache
spring.mustache.content-type
spring.mustache.expose-request-attributes
spring.mustache.expose-session-attributes
spring.mustache.expose-spring-macro-helpers

Each should be deprecated and a replacement with a spring.mustache.servlet.prefix introduced. This will mean that MustacheProperties can no longer extend AbstractTemplateViewResolverProperties. Once these changes are in place, we can tackle #28858 by introducing a reactive-specific property that configures the media types supported by a org.springframework.boot.web.reactive.result.view.MustacheViewResolver.