Add updated_at datetime to billable users blocks

What does this MR do and why?

It updates the dashboard UI to show when billable members stats have been updated, making it clearer that these stats aren't in realtime.

Screenshots or screen recordings

before	after

How to set up and validate locally

• Login as root
• Make sure your instance has a license
• Navigate to admin dashboard (e.g. http://gdk.test:3000/admin)
• Notice the 4 blocks in the license overview have "Update date-time-string" gray text.

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 #375641 (closed)