Fully shard clusters

Sharding keys for clusters were added and backfilled via background migration in #553452 (closed).

Once the background migration has been finalised, add a constraint that one of project_id, group_id or organization_id is present.

Milestone %18.5 (required stop)

• Identify the source of num_nonnulls(organization_id, group_id, project_id) <> 1
• Add a NOT NULL (NOT VALID) constraint on the table's sharding key
• Backfill rows with sharding key

Milestone %18.6

• Finalize sharding key backfill
• Set a NOT NULL constraint on the table's sharding key
• Verify the NOT NULL constraint is present on the table's sharding key for .com