Add NOT NULL on sharding key for merge_requests_closing_issues

What does this MR do and why?

Add NOT NULL constraint on the merge_requests_closing_issues sharding key. The sharding key will be used to determine what Organization the row belongs to.

• Backfill has been finalized in !177644 (merged)
• No presence: true check is added in the model because we're also populating the column with a "BEFORE INSERT OR UPDATE" trigger.
  • Trigger added in !157186 (merged)
  • 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.
• The migration exceeds the 100ms timing guideline at 2482.41ms -- but it doesn't look significant enough to warrant async validation as it only adds a couple of seconds to the total migration time (?)
• 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.

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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