Remove id from create in testing migrations docs

What does this MR do and why?

Update documentation for testing migrations to remove setting id manually.

Testing best practices recommends against this

MR acceptance checklist

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

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A