Update UI to display last updated datetime and/or a note about delayed updates

Proposal

Over in Streamline billable users calculations (#338253 - closed) issue, we discussed how to split that issue out into smaller separate tasks. This issue is one of them where we suggest that we should update the admin dashboard and or user statistics views to include a notice about when the data was last updated and/or that the data is not updated in realtime.

For example,

User statistics are updated every 90 minutes. Last update: 2022-09-28 16:20

Estimate

An estimate was provided previously during the breakdown and refinement of the parent issue. weight 1

Designs

#377389 (closed)

Note: only the date and time additions are relevant. Text changes on the user count descriptions and headings are not part of this implementation.