Search::IndexRepairService still using Repository index for projects

What does this MR do and why?

index_name was set to Repository.__elasticsearch__.index_name which is wrong when projects use a separate index. So replaced it with Project.index_name which gives the correct value based on the status of the migration migrate_projects_to_separate_index

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Before	After

How to set up and validate locally

Make sure the elasticsearch is enabled

1. Open rails console: rails c
2. Get an existing project: p = Project.last
3. Kick off the repair service: Search::IndexRepairService.execute(p)
4. Verify in the logs/elasticsearch.log that the project is queued for indexing and the following log is not present.

{
  "severity": "WARN",
  "time": "2023-10-25T19:22:51.599Z",
  "class": "Search::IndexRepairService",
  "message": "project document missing from index",
  "namespace_id": 71,
  "root_namespace_id": 71,
  "project_id": 28
}
{
  "severity": "DEBUG",
  "time": "2023-10-25T19:22:51.608Z",
  "class": "Elastic::ProcessBookkeepingService",
  "redis_set": "elastic:incremental:updates:0:zset",
  "message": "track_items",
  "count": 1,
  "tracked_items_encoded": "[[97,\"Project 28 project_28\"]]"
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #429456 (closed)