[gem update] use kubeclient 3.0

We'd like to enable tests for kubeclient in debian and only version 3.0 supports webmock 3.0. So we'd like to update kubeclient to 3.0 in debian. Since gitlab is the only package that depends on kubeclient currently, we'd like to update it only when gitlab also bump the dependency. I have also asked them to declare SemVer compliance. If they do, the ~> 3.0 would be better than ~> 3.0.0.