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
@ulferts @NobodysNightmare How should we handle attributes that are added by common plugins (e.g. `spentTime` from costs)? I think we should document them - probably here too, but differentiated somehow.