Deprecate http.mappers.* properties

The http.mappers.* configuration properties assumed that the mapping
was JSON (on of the property names was jsonPrettyPrint) and also only
exposed a small subset of the configuration options supported by
Jackson (and GSON). The property names implied that it would configure
all HTTP mapping, however it was ignored by GsonAutoConfiguration.

This commit deprecates the support for http.mappers.* in favour of
configuring Jackson or Gson instead. Jackson can be configured
declaratively using the spring.jackson.* properties or programtically.
Gson can be configured programatically by using a GsonBuilder to
create a Gson instance with the desired configuration.
gh-1946 has been opened to add support for declarative configuration
of Gson.

Closes gh-1945