Add maintainer step to stable branch template
What does this MR do and why?
Add maintainer step to stable branch template. Highlight that only one maintainer approval is required when backporting bug fixes into a stable branch.
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.