samples coap_server: Move from native_posix to native_sim

* In the sample doc, replace native_posix references with native_sim
* Provide overlays for native_sim
* Enable native_sim in the sample yaml
* Remove native_posix from the sample yaml platform_allow as it was
  filtered out by the host libC

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>