[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'd just not mention that... From a client perspective we are complete here:
- the lockversion can be read
- the client sends the lockversion
- he always receives HTTP 200, but is prepared for HTTP 409

That the server will not currently fire a 409 is sad, but probably should not be mentioned in the specs (as opposed to places where the client would break if it relied on the spec).