Upgrade to Gemnasium v5

What does this MR do and why?

• Upgrade Dependency Scanning CI template to Gemnasium v5.
• Update Dependency Scanning docs to reference :5 images.

This change is part of our annual major bump and deprecation of previous major as announced in https://docs.gitlab.com/ee/update/deprecations.html?removal_milestone=17.0#secure-analyzers-major-version-update

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Related to #455488