Clarify administrator user permissions

What does this MR do and why?

This clarifies that GitLab Admins can approve/reject deployments by default. It emanates from this ticket where the customer found out some users were unexpectedly allowed to approve deployments. We later discovered that the said users were all GitLab Admins.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.