Backfill zoekt replica on zoekt indices

What does this MR do and why?

With the introduction of Zoekt::Replica, this backfills all existing zoekt indices that don't already have a replica assigned and assigns them one.

Related to #467403 (closed) and &14188

Query Plan

UPDATE "zoekt_indices" SET "updated_at" = '2024-06-29 21:12:13.217486', "zoekt_replica_id" = 1 WHERE "zoekt_indices"."id" = 2

Time: 4.759 ms  
  - planning: 1.010 ms  
  - execution: 3.749 ms  
    - I/O read: 0.855 ms  
    - I/O write: 0.000 ms  
  
Shared buffers:  
  - hits: 50 (~400.00 KiB) from the buffer pool  
  - reads: 1 (~8.00 KiB) from the OS file cache, including disk I/O  
  - dirtied: 1 (~8.00 KiB)  
  - writes: 0  

https://postgres.ai/console/gitlab/gitlab-production-main/sessions/29561/commands/91787

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