Add NOT NULL for sharding key on incident_management_issuable_escalation_statuses

What does this MR do and why?

Adds NOT NULL constraint on sharding key column project_id on table: incident_management_issuable_escalation_statuses.

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 incident_management_issuable_escalation_statuses.

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