Move install step for pg_trgm to later in the setup process.

What does this MR do?

The step for Installing the pg_trgm extension for PostgreSQL is not possible at the stage where it's currently listed in the setup process. We have no access to the RDS instance (by AWS design) and the GitLab instance doesn’t exist yet so we have no way of connecting to the database. This can only be done once we have GitLab installed on an EC2 instance.

This MR corrects the order of the setup steps.

Related issues

See #197154 (closed)

Author's checklist

• [ x ] Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• [ x ] Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.