Handle expired certificates
KDE Connect is 10 years old! Which means that device certificates from early users might begin to expire, because 10 years is the lifetime we set when generating them [1].
Currently, once a certificate expires KDE Connect will stop working without an explanation (maybe something is printed on the logs?) and the configuration will need to be deleted manually to fix it. This is confusing and we could handle it better.
Since this is a rare thing to happen, we don't need to worry about adding a UI message explaining this or anything. The simplest solution is to delete and re-create the certificate when we detect it is expired. We could also auto-unpair peer devices whose certificates are expired.
[1] In the Qt client and Android client. On the iOS client it's not explicitly set, which I think means it doesn't expire.