Add update notification for admin users

### Description

Looking at our telemetry data, we have a fairly long tail of self-hosted instances that aren't being kept up to date.

The proposal here is to add a banner to the admin pages to notify an admin user that there is an update available.

Given our regular release cadence, we can implement this without needing a ping back to get the latest version - so it would work for air-gapped installs as well. AIUI, this is what gitlab does.

1. Hardcode the date/version of a particular release.
2. Calculate how many release cycles (28 days) have happened since that date
3. Calculate how many releases ahead the platform is compared to that known version
4. Display a banner if, by the above calculations, the platform is not on the latest.

Other options:
- update release process to update the hardcoded date/version with each release - then we just need to calculate if we're 28 days past that date
- do a get of the npm metadata for the ff package - although that requires network access.

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate