Enable storage purchase via GitLab by default

What does this MR do and why?

The new_route_storage_purchase feature flag has been enabled on production for 5 days now. This MR default enables the feature flag.

Related: #327896 (closed)

Screenshots or screen recordings

storage_small

How to set up and validate locally

1. Run CustomersDot locally
2. Create a new group
3. Navigate to Settings -> Usage Quotas -> Storage tab and click Purchase more storage
4. Complete the flow

Fast-forward link ➡ http://localhost:3000/-/subscriptions/buy_storage?selected_group=<id_of_an_existing_group>

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.