feat(renovate): track oldest maintained k8s

[kiblik]

This is duplicity of #13545. But as soon as the config in master and dev is not the same, we can see Warnings like:

Some dependencies could not be looked up. Check the Dependency Dashboard for more information.

And

Renovate failed to look up the following dependencies: Failed to look up custom.endoflife-oldest-maintained package kubernetes.
Files affected: .github/workflows/k8s-tests.yml

By merging this to the master sooner, warnings should disappear.

[mtesauro]

Approved