Add background migration to delete orphaned sbom_occurrences

What does this MR do and why?

This MR implements a batched background migration to delete sbom_occurrences records with a project_id that belongs to a non-existent project. There are some records created in 2024 that are in this state.

References

Relates to: #541931

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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