Add index on zoekt repositories for schema checks

What does this MR do and why?

This change adds a new database index to improve query performance when searching for repositories in the Zoekt search system. The index is created on three columns (project_identifier, state, and schema_version) of the zoekt_repositories table, which will make it faster to find repositories based on these specific criteria. The migration is set up to run safely without blocking other database operations, and includes the proper rollback functionality to remove the index if needed.

Query Plans

Before

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/43106/commands/131783

After

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/43106/commands/131785

References

https://gitlab.slack.com/archives/C07SBRDSJUS/p1756911235940409?thread_ts=1756818977.421629&cid=C07SBRDSJUS

Screenshots or screen recordings

Before After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Edited by John Mason

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