Adding warning note to template

What does this MR do and why?

Related to #379030 (closed)

The purpose of this MR is to add a warning note into this template so that it is clear to external users that this template is intended for GitLab internal use only and will likely fail. If they wish to use DAST templates, they should consider using DAST.gitlab-ci.yml and DAST.latest.gitlab-ci.yml for their purposes.

Describe in detail what your merge request does and why.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.