[APIv3] Specify first iteration of WorkPackage PATCH-Endpoint

This PR belongs to [#16770](https://community.openproject.org/work_packages/16770).

It defines the very first portion of the `PATCH` endpoint for work packages.
I think that I would not be okay with that.

Assume a client wants to set a property like `id` or `createdAt`. Those are not writable now and will never be. A client trying to set them obviously does not know this.

I therefore vote for returning an error. Because the request was not **OK** (HTTP 200), it was *bad* and we could not (fully) fulfill it. And requests that can't be fulfilled completely should not be fulfilled at all, because this could lead to inconsistent state (assuming the client really meant to change a read-only field).