Add `deleted` event for merge request webhook
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## Problem to Solve
Merge requests support the following events:
- open
- close
- reopen
- update
- approved
- unapproved
- merge
This is missing an event for `deleted` which could happen to an MR. For systems relying on events they would suddenly have information on an MR that no longer exists and not be able to take any action.
## Proposal
An additional webhook event should be sent for when a merge request is deleted.
issue