Change permissions on Retrieve/Update/PartialUpdateWorkerVersion

https://redmine.teklia.com/issues/8856

PartialUpdateWorkerVersion and UpdateWorkerVersion require using Ponos task authentication. They used to require is_internal before we removed that in #1467 (closed). This was intended for the Git repository import processes, which we have removed months ago.

RetrieveWorkerVersion does not require anything at all. No authentication and no permissions.

Instead, RetrieveWorkerVersion should use Worker.objects.executable as all the other endpoints do, and PartialUpdateWorkerVersion and UpdateWorkerVersion should require admin access to the worker.