Add NOT NULL for sharding key on vulnerability_finding_evidences

What does this MR do and why?

Add NOT NULL constraint to vulnerability_finding_evidences.project_id column.

• We do not add a validation on the model because we're populating the column using a "BEFORE INSERT OR UPDATE" trigger.
• We have finalized the backfill migration for this column.

The sharding key will be used to determine what Organization the row belongs to.

No follow up issue is created to finalize the async validation because this will be tracked in #493768 (closed)

References

• Organization Isolation and Sharding Keys: https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/organization/isolation/
• Sharding Key (Cells) Development Guide: https://docs.gitlab.com/ee/development/cells/#defining-a-sharding-key-for-all-cell-local-tables
• Related to #493768 (closed)

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.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A