Prepare ASYNC check constraint for NOT NULL sharding key on redirect_routes

What does this MR do and why?

Addresses #561339

This MR is the final step to fully shard the table:

• finalize sharding key backfill
• finalize oprhan row removal
• prepare ASYNC check constraint for NOT NULL sharding key

References

• verify 0 records with NULL sharding key -> https://postgres.ai/console/gitlab/gitlab-production-main/sessions/43850/commands/134488
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