Draft: Add sharding_key `namespace_id` to achievements

This table has been automatically identified as a cell local table.

All cell local tables require a sharding key.

The sharding key has been automatically identified from one of the existing columns because it is a NOT NULL column referencing either projects or namespaces.

Please confirm that this table should indeed be cell local, and that the selected sharding key is appropriate.

If you have any questions or concerns, reach out to @tigerwnz or @DylanGriffith.

This merge request was created using the gitlab-housekeeper gem.