Use StatisticsCard in usage_quotas/seats

What does this MR do and why?

It uses the StatisticsCard component added in !80261 (merged) in usage_quota seats app (group's settings > usage quota > seats).

MR	status
!80261 (merged) Add statistics card component	✅ merged
!82497 (merged) Adjust statistics card component	✅ merged
!80260 (merged) Add statistics seats card	✅ merged
!82399 (merged) Use StatisticsCard in usage_quotas/seats ​	👈 you're here

Screenshots or screen recordings

before	after (paid subscription)	after (free subscription)

How to set up and validate locally

• Navigate to a group's settings > usage quotas > seats 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 #350788 (closed)