Add vulnerability_finding_due_dates table to store remediation due dates

What does this MR do and why?

Add vulnerability_finding_due_dates to store remediation due dates for vulnerability findings.

Changes

• Create vulnerability_finding_due_dates table with:

  • vulnerability_occurrence_id (unique, FK -> vulnerability_occurrences, ON DELETE CASCADE)
  • project_id
  • due_date
  • timestamps

• Add indexes on vulnerability_occurrence_id and project_id

• Configure Loose Foreign Key cleanup for project_id → projects

• Add Vulnerabilities::FindingDueDate model and has_one :finding_due_date association on Vulnerabilities::Finding

• Add factory and model specs

• Include association in import/export configuration

References

Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/592222+

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