Dependency Scanning: Simplify Poetry workaround

What does this MR do?

Follows up on !47940 (merged) & victor-engmark/gitlab!1 (closed) to simplify our workaround to missing Poetry support. Prepared by @victor-engmark, I just rebased.

Things to consider: Does this simplification remove any relevant details for other packaging systems? After all, that trouble-shooting section should be generalisable, with Poetry being just a popular example for the whole approach.

How to setup and validate locally

Example below:

1. Start with a Python project
2. Enable dependency scanning
3. Add the new code to .gitlab-ci.yml
4. Verify that the gemnasium-python-dependency_scanning job runs successfully.

Related issues

#7006 (comment 640545230)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• [-] Ensure that the product tier badge is added to doc's h1.
• Request a review based on the documentation page's metadata and associated Technical Writer.

To avoid having this MR be added to code verification QA issues, don't add these labels: feature, frontend, backend, ~"bug", or database

Review checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
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