Fix the corrupted `scanner_ids` of `vulnerability_reads`

What does this MR do and why?

There was a bug in production where the scanner_id of a vulnerability finding would be updated, yet the scanner_id of the related vulnerability_read would remain unchanged. This was an issue as the vulnerability_read should always be in sync with the vulnerability_finding.

The bug has already been fixed in production, so now we need a migration to fix all of the vulnerability_reads that were affected by the bug.

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Changelog: fixed
EE: true

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MR acceptance checklist

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Related to: #454290 (closed)
resolves: #454827 (closed)