Validate NOT NULL on merge_request_assignees sharding key

What does this MR do and why?

Validate the NOT NULL constraint for the sharding key on merge_request_assignees

Also updates the desired_sharding_key entry to sharding_key in the table's dictionary as adding the constraint is the final requirement for it to be a fully qualified sharding key.

• Async validation is verified to be complete on postgres.ai via \d+ merge_request_assignees (no shareable link)
• No presence: true validation on the model because the column is also populated with a "BEFORE INSERT OR UPDATE" trigger.
  • Trigger: https://gitlab.com/gitlab-org/gitlab/blob/f4d36df20b63a03ad05013e18f553ba9846c8467/db/structure.sql#L1918-L1932
  • We have no well-defined plan yet on the removal of triggers but we will make sure that the application code is setting a the value on the column BEFORE removing the trigger.
• Applied pipeline:skip-check-migrations as reverting this migration is intended to be #no-op. Rolling back wouldn't restore structure.sql to its previous state.
  • Failed spec: https://gitlab.com/gitlab-org/gitlab/-/jobs/9087528149

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

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Screenshots or screen recordings

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How to set up and validate locally

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