Send status messages about security MR merges

What does this MR do and why?

When security merge requests get merged, it sends a slack message notifying the release manager with the status and job url of the merge job.

This is a followup to !3104 (merged) where the early merge chatops command was automated in the security pipeline. Now when the job to merge succeeds or fails, it will send the RM status message, similar to:

Related: gitlab-com/gl-infra/delivery#20189 (closed)

The slack notification is behind a feature flag merge_slack_notification.