Add NOT NULL for sharding key on design_management_versions

What does this MR do and why?

Add NOT NULL constraint to design_management_versions.namespace_id column.

• Backfill was finalized in !171611 (merged)
• This is the last step towards the desired sharding key migration where we add a NOT NULL constraint and update the database dictionary accordingly.
• namespace_id is guaranteed to be populated by !186022 (diffs) and/or the trigger_96a76ee9f147 database trigger.

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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