Merge the TaskDetailsFromAgent and TaskUpdate API views

https://redmine.teklia.com/issues/7517

The TaskUpdate API view provides UpdateTask and PartialUpdateTask, which only allow updating a task's state to stopping or pending. stopping causes a single task to stop and pending causes a task to restart. In the frontend, stopping and restarting a single task (instead of stopping and retrying a whole process) is only provided to instance admins, but any process admin can actually call the API endpoints.

The TaskDetailsFromAgent API view provides RetrieveTaskFromAgent (the only way to retrieve a single task, used by both agents and the frontend), UpdateTaskFromAgent (never used) and PartialUpdateTaskFromAgent (to update a task's state from an agent).

RetrieveTaskFromAgent would be better named just RetrieveTask since it is used not just by agents. Both of the update endpoints only allow updating a task's state, but they just allow it with different rules. We could merge both API endpoints to make them easier to understand and maintain.