Integration of new `StatusCreatedAt` GSI in web application

[srtalbot]

Context on why we had to create a new GSI: https://github.com/cds-snc/platform-forms-client/issues/4205#event-14308835557

What has already been implemented https://github.com/cds-snc/platform-forms-client/pull/4257

What is left to do:

• Write a migration script to populate this new field for all existing Vault items, which will also update the new GSI.
  • Script should initialize Status#CreatedAt (in NAME# items) and CreatedAt (in CONF# items) fields if undefined
• Adjust the existing query in the Forms App to use the new StatusCreatedAt GSI and perform thorough testing.
• Deploy the adjustments to the Forms App.
• Drop the old GSI after confirming the new setup works as expected.

[anikbrazeau]

Related issue: https://github.com/cds-snc/platform-forms-client/issues/3051