Fully shard protected_branch_push_access_levels

Follows up on !175714 (merged) in light of Sharding Priorities.

Milestone 18.6

• Prepare to fully shard table -> !209876
  • add NOT NULL (NOT VALID) constraint on the table's sharding key
  • prepare ASYNC constraint check on NOT NULL sharding key
• Verify the NOT NULL (NOT VALID) -> NOT NULL constraint has changed on the sharding key on .com
• Fully shard table ->
  • update db/docs/*.yml with sharding key
  • SYNC validate NOT NULL constraint on sharding key
• verify the NOT NULL constraint check is set on the table's sharding key