Docs: Update "Gemnasium DB" to "GitLab Advisory Database"

What does this MR do?

This MR renames "Gemnasium database" and "Gemnasium DB" to "GitLab Advisory Database" in docs as part of an effort to name and describe the database consistently (&7798 (closed)).

I did not complete invasive edits within the Dependency Scanning documentation. There, "the Gemnasium database" can be read to refer to the database used by the Gemnasium analyzer. I didn't want to make things more confusing by mixing GitLab Advisory Database into that content.

Related issues

Epic: Align Advisory Database naming and framing (&7798 - closed)

Specific issue: Update docs/blogs from "Gemnasium DB" or inform... (#357057 - closed)

Author's checklist

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If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
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