Specify patchable dates

## Work Packages

* https://community.openproject.org/work_packages/17417
* https://community.openproject.org/work_packages/17418

## Changes

* clearer wording regarding time related types
* Project ressource now uses `createdAt` and `updatedAt` instead of `createdOn` and `updatedOn`
* Dates are now patchable
I would rather document them in the plugins. In the worst case, we have two plugins adding the same attribute name but those having different meanings, e.g values. Additionally, I wouldn't want OpenProject to know to much about the plugins.

FWIW, ```spentTime``` is an attribute of OpenProject itself. Costs adds ```spentHours``` (for now: https://github.com/finnlabs/openproject-costs/pull/114)  and others.