Add NOT NULL for sharding key on release_links

What does this MR do and why?

Add NOT NULL constraint to release_links.project_id column.

• We do not add a presence: true check in the model because we're populating the column with a "BEFORE INSERT OR UPDATE" trigger. When new objects a created (or updated), the sharding key attribute might be nil but will populated on the database operation before being committed.
• We have finalized the backfill migration for this column.

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

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)

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