Backfill release_date for unreleased ai_catalog_item_versions records

What does this MR do and why?

There are currently 59 records in the ai_catalog_item_versions table where release_date is NULL. To support new validations #566006 and #566007, we need to backfill these missing release_date values.

This merge request (MR) adds a migration to backfill those records.

What does this MR do?

This migration updates the release_date of each item version to match its created_at date.

Impact

• Currently, there are 59 records in production that have blank release_date
• This migration will update only those records.
• ai_catalog_item_versions is small tables, and as shown in the query plan, the execution time is minimal.

Query plans

• Migration in this MR : https://console.postgres.ai/gitlab/gitlab-production-main/sessions/44981/commands/138072
• Impacted records : https://console.postgres.ai/gitlab/gitlab-production-main/sessions/44981/commands/138073

Related discussion

• #566006 (comment 2825411911)
• #566006 (comment 2824377933)

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.

Related to #566006