Fully shard redirect_routes

RedirectRoutes (# rows: https://postgres.ai/console/gitlab/gitlab-production-main/sessions/41965/commands/128840)

This issue is a continuation of #514146 (closed)

Milestone 18.3

• set NOT NULL NOT VALID on sharding key -> !198933 (merged)
• backfill sharding key -> !199189 (merged)
• backfill sharding key 100% complete

Milestone 18.5 (required stop)

• delete 24 orphan rows that point to projects that have been deleted -> !204393 (merged)
• verify 0 records with NULL sharding key -> https://postgres.ai/console/gitlab/gitlab-production-main/sessions/43850/commands/134488

Milestone 18.6

• Prepare to fully shard table -> !197966 (merged)
  • finalize sharding key backfill
  • finalize oprhan row removal
  • validate ASYNC NOT NULL on sharding key
  • update db/docs/*.yml with sharding key
• verify NOT NULL (NOT VALID) -> NOT NULL on .com after ASYNC validation
• Fully shard table -> !211260 (merged)
  • validate SYNC NOT NULL on sharding key
  • update db/docs/redirect_routes.yml
• verify the NOT NULL constraint check is set on the table's sharding key on .com

Milestone %18.7

• Fix #581676 -> !215320 (merged)
• Backport fix to %18.6 -> !215588 (merged)
• Fully shard table for self-managed instances -> !215923
  • delete orphan rows where where(source_type: 'Namespace', namespace_id: nil)
  • validate sharding key constraint for all instances except .com