Reduce Update Status module's nagging messages on admin/* pages

Description of the need

The Update Status module displays error messages on all administrative pages when security updates are available, such as on the /admin/modules page:

This behavior has been in place since Backdrop 1.0, something we carried over from Drupal 7.

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Backdrop now has several other places where administrators are notified of updates, including:

1. The admin toolbar Notification icon (from #639)

2. The admin dashboard:

3. The status report:

4. And the new unified updates page added in https://github.com/backdrop/backdrop-issues/issues/2714

And although we are working on fixing some performance impacts caused by Update module in https://github.com/backdrop/backdrop-issues/issues/5046, displaying these messages requires that Update module perform relatively expensive comparison operations that can cause admin pages to slow down dramatically. We could further eliminate expensive processing on random administrative pages if we were to stop showing these messages. Instead we could relegate regenerating the available updates list to more predictable locations, such as the status report and on cron jobs.

Proposed solution

Remove the nagging messaging Update module currently displays on all /admin/* pages, instead allow other existing update mechanisms (email notifications, Twitter, the Backdrop Security feed, the notification dot, status report, and dashboard) be considered adequate notification of security updates.

Alternatives that have been considered

• None yet.

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PR: https://github.com/backdrop/backdrop/pull/3638