Upgrade to Gemnasium v4

What does this MR do and why?

• Upgrade Dependency Scanning CI template to Gemnasium v4.
• Update Dependency Scanning docs to reference :4 images.

Screenshots or screen recordings

CI template tested using gitlab-org/security-products/tests/csharp-nuget-dotnetcore!47 (closed)

See https://gitlab.com/gitlab-org/security-products/tests/csharp-nuget-dotnetcore/-/pipelines/854631969

How to set up and validate locally

In a project supported by Dependency Scanning, switch to this CI template, run a pipeline, and check the Dependency Scanning job.

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.

Related to #408528 (closed)