Updates to the definition of done to help reduce production outages

What does this MR do and why?

Updates to the definition of done per gitlab-com/www-gitlab-com#12302 (closed) to clarify in order to help reduce the chances of production outages.

Screenshots or screen recordings

Review app: http://main-ee-70528.178.62.207.141.nip.io/ee/development/contributing/merge_request_workflow.html#definition-of-done

How to set up and validate locally

Check the review app

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.