Remove hardcoded buy storage url

What does this MR do and why?

New test case from upstream in namespace_helper introduced a hardcoded URL which should be replaced.

Screenshots or screen recordings

Backend change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.