Adjust vulnerability backup model foreign keys

Summary

While implementing the tracking vulnerabilities on the multiple branches feature, some of the model schemas have been changed. An example is the Vulnerabilities::StateTransition model. This model was associated with the Vulnerability model, but it's going to be associated with the Vulnerabilities::Finding model once the feature is complete. This requires the backup_vulnerability_state_transitions table schema to be updated, as it currently has the vulnerability_id foreign key.

Affected Tables

After investigation, the following 6 backup tables need to be updated to include finding_id column mapping for vulnerability_occurrence_id:

Backup Table	Backup Model	Source Model
backup_vulnerability_state_transitions	Backups::VulnerabilityStateTransition	Vulnerabilities::StateTransition
backup_vulnerability_severity_overrides	Backups::VulnerabilitySeverityOverride	Vulnerabilities::SeverityOverride
backup_vulnerability_issue_links	Backups::VulnerabilityIssueLink	Vulnerabilities::IssueLink
backup_vulnerability_merge_request_links	Backups::VulnerabilityMergeRequestLink	Vulnerabilities::MergeRequestLink
backup_vulnerability_external_issue_links	Backups::VulnerabilityExternalIssueLink	Vulnerabilities::ExternalIssueLink
backup_vulnerability_reads	Backups::VulnerabilityRead	Vulnerabilities::Read

Implementation

For each table:

1. Add database migration to add finding_id bigint column to the backup table
2. Update the backup model's column_mapping to include vulnerability_occurrence_id: :finding_id

This follows the existing pattern used by other finding-related backup tables like backup_finding_flags.