Update policies docs to mention merge request pipelines

What does this MR do and why?

This MR updates "Merge request approval policies" documentation to mention the fact that latest security templates are required with merge request pipelines.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #428173 (closed)