Gitlab: close Merge Requests instead of deleting them

[tmeijn]

Describe the bug
Currently when Multi-Gitter is called with the close command, the GitLab implementation actually deletes the Merge Request instead of closing it. This results in the Merge Request not being able to be found anymore. Furthermore, this requires admin or owner level privileges, which is not necessarily required for Multi-Gitter to work.

Expected behavior
Instead of deleting, the Merge Request should be closed

Additional context
The function that should close the Merge Request actually calls the DeleteMergeRequest, which calls the DELETE method on the api

multi-gitter/internal/scm/gitlab/gitlab.go

Line 393 in bb08990

_, err := g.glClient.MergeRequests.DeleteMergeRequest(pr.targetPID, pr.iid, gitlab.WithContext(ctx))

Instead the Merge Request should be updated with the state_event: close

[github-actions]

Included in release v0.39.0 🎉