Fix GitLab Update MR

[EyalDelarea]

• [x] All tests passed. If this feature is not already covered by the tests, I added new tests.
• [x] I used go fmt ./... for formatting the code before submitting the pull request.
• [x] This feature is included on all supported VCS providers - GitHub, Bitbucket cloud, Bitbucket server, GitLab and Azure Repos.
• [x] I added the relevant documentation for the new feature.

Bug description:

We used close/open for state_event object when updating GitLab Merge Request. Which resolved in panic and errors from the api.

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state_event	string	no	New state (close/reopen).

Fix:

Update to valid states, update open -> reopen.

GitLab api docs: https://docs.gitlab.com/ee/api/merge_requests.html#update-mr

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