Selecting MRF policies didn't work as intended any more

Selecting MRF policies didn't work as intended any more

* Policies were put under a new module (Pleroma.Web.ActivityPub.MRF.Policy instead of Pleroma.Web.ActivityPub.MRF), but this wasn't changed in the Pleroma.Web.ActivityPub.MRF @mrf_config_descriptions
* I don't have a unit test to prevent similar problems in the future because I don't find a proper way to do it
    * The descriptions in the unit tests are defined in the unit tests, so if someone changes module names in the code, the tests wont see it
    * The list is generated in Pleroma.Docs.Generator.list_behaviour_implementations, but I can't do a check in the when clause of the function to see if the provided module is a behaviour or not.

Backport of: https://git.pleroma.social/pleroma/pleroma/-/merge_requests/3509