Update security workflow with a link

What does this MR do and why?

This adds a link to the security developer workflow issue template to help engineers discover which versions they need to target with their backports.

Related to gitlab-com/gl-infra/delivery#19837 (closed).

Screenshots or screen recordings

N/A

How to set up and validate locally

You can view the rendered markdown here and see the link works: https://gitlab.com/gitlab-org/gitlab/-/blob/1a6e5bd9c75bf5093c87aac65c0e97de887156d2/.gitlab/issue_templates/Security%20developer%20workflow.md

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.