Change the auto-configure MongoMappingContext to use the
SimpleTypesHolder instance `Set` that's produced by a CustomConversions
bean, which we in turn now default, too.

This update is necessary as `CustomConversions` registers converters by
inspecting the classpath (to automatically detect Java 8, JodaTime etc.)
and by that rendering the types for which we find converters for as
simple ones, i.e. non-entities.

Fixes gh-6881
Closes gh-6882