Updates the stable branch template

What does this MR do and why?

Updates the stable branch template with a step to ensure a severity label is added and to refer to the runbook for engineers and maintainers.

Related to gitlab-com/gl-infra/delivery#2890 (closed)

Requires gitlab-org/release/docs!569 (merged) to be merged first.

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.