Update endpoint for task checking to be a simple update method

**Is your feature request related to a problem? Please describe.**
My feature is related to the unability to "uncheck/unmark" task that was once marked as completed.

**Describe the solution you'd like**
I'd like an endpoint like `/mentorship_relation/{request_id}/task/{task_id}/uncomplete` to be added. I'm also willing to work on it.

**Describe alternatives you've considered**
Alternative solution would be to change the implementation on the Android side. In my opinion, checkboxes are a bit misleading for a user, as they imply that a task can be "unchecked". Example of such a confusing behavior below: