[entities] Restore dc_ methods instead of __getattr__ proxy
In 8de62610cea2, a __getattr__ proxy was introduced to handle compatibility
with dc_* methods being called on entity instances instead of through the
IDublinCore adapter. Unfortunately, __getattr__ does not play well with
super() and since the latter is often used by, we here restore all dc_ methods
with an explicit proxy to the adapter. More verbose but should work.
Closes #17103999.
