Fully shard merge_request_cleanup_schedules

MergeRequestCleanupSchedules (# rows: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/41965/commands/128841)

Milestone 18.3

• add a NOT NULL (NOT VALID) constraint on sharding key for new records -> !199935 (merged)

Milestone 18.4

• add index on foreign key + add a foreign key constraint (NOT VALID) -> !201825 (merged)
• verify the foreign key constraint (NOT VALID) and index are present on the table's sharding key column
• validate foreign key constraint (ASYNC) -> !201986 (merged)
• verify the foreign key constraint is present on the table's sharding key column
• validate foreign key constraint (SYNC) -> !203272 (merged)
• queue sharding key backfill -> !198596 (merged)
• sharding key backfill 100% complete
• verify 0 records with NULL sharding key -> https://postgres.ai/console/gitlab/gitlab-production-main/sessions/43299/commands/132266

Milestone 18.5 (required stop)

Milestone 18.6

• Prepare to fully shard table -> !209477 (merged)
  • finalize sharding key backfill
  • 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 ->
  • SYNC validate NOT NULL constraint on sharding key
  • update db/docs/*.yml with sharding key
• verify the NOT NULL constraint check is set on the table's sharding key