Improve reviewers fetching in Security Merge Requests

As suggested by @joernchen in this discussion, some reviewers unassign themselves from merge requests as soon as their review is done. This prevent the cli from counting them as reviewers because we only use the ones assigned to the MR. We could parse the notes of the MR to fetch review requests, like for this one: https://gitlab.com/api/v4/projects/gitlab-org%2fsecurity%2fgitlab/merge_requests/1881/notes (internal link).