Move `namespace_setting` validations to standard rails validations

What does this MR do and why?

This is a follow up suggested in !153099 (comment 1915994451)

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

Pipelines passing should be enough validation