Graduate update_storage_usage_design feature flag

What does this MR do and why?

Releases the feature flag update_storage_usage_design, which controlled a new design for Usage Quotas > Storage tab for free namespaces.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Ensure you don't have update_storage_usage_design feature flag enabled locally Feature.disable(:update_storage_usage_design)
2. In a free namespace, visit Usage Quotas > Storage
3. Verify it shows the new design

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.

Related to #358296 (closed)