Notify when (some) runners are out of date/incompatible

Description

We just spend some hours trying to figure out why our pipelines didn't work any more. It turned out that our runners were hopelessly outdated.

Proposal

It would be nice if somewhere -- e.g. on top of the CI/CD pages -- a notification was shown, telling you that "Runner A, B and C are running version X which is incompatible with GitLab version Y."