Update DAST On-Demand Scan Template

What does this MR do and why?

Describe in detail what your merge request does and why.

This Merge Request address the following requirement of the issue :#327070 (closed)

As part of on-demand scans, users can run an API scan through a web interface. The On-demand process uses the OnDemand YAML file https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Security/DAST-On-Demand-Scan.gitlab-ci.yml.

When the API Security (Peach DAST scanner) tool is ready to roll out as the default API scanner, the template above should be updated to use the API Security DAST scanner instead of the zap engine.

The new scanner tool is not ready yet.

We should use a Feature Flag and keep it disabled until the new scanner tool is ready to roll out.

Screenshots or screen recordings

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How to set up and validate locally

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

1. Enable the Feature Flag dast_api_scanner

rails c
Feature.enable(:dast_api_scanner)

2. Create a new DAST On-Demand scan

   • Choose a project
   • Go to Security & Compliance > On-demand Scans
   • Click on the New DAST scan

3. Create a new Site Profile selecting Rest API as Site Type

4. Create a new On_demand DAST Scan using the site profile created in the previous step

5. Click in save and run scan

MR acceptance checklist

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