Update seat_usage references

What does this MR do and why?

It renames and moves the seats_usage folder to the usage_quotas/seats folder as part of unifying usage quotas pages &7176 (closed).

Screenshots or screen recordings

There are no visual changes.

How to set up and validate locally

In a root group with a paid plan, from the left sidebar go to Settings > Usage Quotas and observe the seats usage tab.

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 #346533