Add NOT NULL for sharding key on issue_emails

What does this MR do and why?

Adds NOT NULL constraint on sharding key column namespace_id on table: issue_emails.

This merge request is the last step towards transforming the desired sharding key into a sharding key.

This involves the following changes:

• Adding a not null constraint for the sharding key column (namespace_id).
• Setting namespace_id as sharding_key of issue_emails.

We are skipping model-level validations when adding NOT NULL for now: !189213 (comment 2480466972)

References

!189213 (comment 2480466972) https://docs.gitlab.com/ee/development/database/multiple_databases.html#defining-a-sharding-key-for-all-cell-local-tables

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