feat: Add "expires after" to CertificateRenewal alert description

[Kumm-Kai]

Currently, the alert description does not include when the certificate will expire:
Certificate for "example.org" should be renewed in Kubernetes secret "foo/bar-tls"

After this change:
Certificate for "example.org" should be renewed as it expires after 5d 22h 39m 55s in Kubernetes secret "foo/bar-tls"

[npdgm]

There was no time left in alert descriptions on purpose. Because depending on the Alertmanager sink and how alerts are consumed, the information may not get updated after the alert is fired. Rendering a duration is not a concern when alerts are forwarded as email or to instant messaging, but it can be misleading when alerts are displayed on a dashboard for example.
However it seems that Karma is refreshing from Alertmanager and would not have this issue now. And that's what most people use. So let's add the duration and not worry about very limited edge cases.
Thanks a lot!