Rename spring.mvc.converters.preferred-json-mapper to spring.http.converters.preferred-json-mapper

[wilkinsona]

spring.mvc.converters.preferred-json-mapper is used by HttpMessageConvertersAutoConfiguration. It controls the HttpMessageConverter for json that's auto-configured (Jackson, Json-B, or Gson) and affects anything that uses HTTP message converters (for example, RestTemplate and RestClient) not just Spring MVC. We should rename it to spring.http.converters.preferred-json-mapper.