Work item references also work on merge request closing pattern

We can close issues automatically when an MR is merged. For this you need to reference an issue on the MR's description using the closing pattern (closes, fixes, etc or what is configured as the closing patter regex). This should also work if you reference a work item using it's full URL as in the following example

Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/464632

Group level work item references should also work as they currently don't