Add check_organization_id_nullness constraint check for project/group runner tables

Once Finalize organization_id backfill migrations (#523850 - closed) is done, we need:

• a PDM to add a check_organization_id_nullness check constraint to the group/project partitions, and also to the routing tables, in the same way was check_sharding_key_id_nullness.
• to modify db/docs/* files to point sharding_key to organization_id: organizations.
• Remove Ci::Runner#ensure_organization_id

Documentation: https://docs.gitlab.com/development/database/not_null_constraints/#add-a-not-null-constraint-to-an-existing-column (step N.M+1)