Authors can merge their own MRs in some situations

What does this MR do and why?

• The rationale for not allowing MR authors to merge their own work is that "we are trying to prevent is a singular person making a change into production".
• This is already prevented because it is not possible to approve an MR where you added one or more commits. And, an MR must be approved by the appropriate groups/codeowners before it is mergeable. Therefore, an MR author cannot unilaterally merge their own MR in any situation.
• "MR authors with proper approvals can set merge on their own MR" is better for efficiency. If you search in Slack: in:#backend_maintainers MWPS - you will see that multiple times a day engineers are asking other people to merge for them. This is inefficient and ultimately a check-the-box action rather than a real security assurance because the MR already has the required approvals.
• See more detail in conversation here: gitlab-org/quality/triage-ops!2272 (comment 1419061593)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.