Uses non-metadata severity

What does this MR do and why?

Uses severity attribute when severity isn't part of raw_metadata.

Link to sentry

Related issue: #427639 (closed)

Example of one record from production:

[ gprd ] production> Vulnerabilities::Finding.find(1984925548).metadata['severity']
=> nil
[ gprd ] production> Vulnerabilities::Finding.find(1984925548)['severity']
=> "high"

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.