Rename subscription feature category

What does this MR do and why?

Replaces subscription_usage_reports with subscription_cost_management to align with feature category changes made in gitlab-com/www-gitlab-com!116198 (merged).

Screenshots or screen recordings

N/A

How to set up and validate locally

This is all for tooling stuff, so if the pipeline passes we're good 👍

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.