Resolve "Backfill vulnerability_statistics.traversal_ids, archived"

What does this MR do and why?

Implement a background migration that fills in the archived and traversal_ids for existing vulnerability_statistics records from the respective owning project and namespace.

NOTE: We should sync these columns before backfilling them, so this MR depends on the following ones:

• Update vulnerability_statistic.archived when pr... (!177990 - merged)
• TODO: MR for syncing traversal_ids

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #513030 (closed)