Document RETIREJS_ADVISORY_DB_INSECURE DepScan var

What does this MR do?

This MR documents the new RETIREJS_ADVISORY_DB_INSECURE variable and behaviour change for ADDITIONAL_CA_CERT_BUNDLE added to dependency scanning in gitlab-org/security-products/analyzers/retire.js!32 (merged).

Related issues

#213256 (closed)

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• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
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