Backfill vulnerability_occurrence_id to finding associations

What does this MR do and why?

As part of the vulnerabilities on multiple branches project, we are changing the relations for many models to be related to vulnerability_occurrences instead of vulnerabilities. In !214280 (merged), we began setting the vulnerability_occurrence_id for new records. This MR backfills the data for existing records.

Tables not included in this migration:

• sbom_occurrence_vulnerabilities - We are not setting this ID during ingestion yet.
• vulnerability_user_mentions - We still need to figure out how we are migrating this table.

References

Relates to: #556203 (closed)

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How to set up and validate locally

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