Add NOT NULL constraint to ci_runner_namespaces.namespace_id

What does this MR do and why?

This MR completes the implementation of #446169 (closed) in %17.2 by adding NOT NULL constraint to ci_runner_namespaces.namespace_id. The previous MRs in %17.1 already validated that no new records could be created with nil values, and deleted invalid records from the database.

This MR also removes the "relaxed sharding key" exception in spec/lib/gitlab/database/sharding_key_spec.rb

Changelog: changed

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