Optimise keyset pagination for audit events

What does this MR do and why?

Optimise keyset pagination for audit events

This commit add created_at to keyset ordering to enable partition pruning

EE: true

References

Query:

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/45220/commands/138706

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 #493723