Update KubeOne CI jobs (#1237)

* Remove 1.16 and 1.17 conformance and upgrade tests

* Update Golang image to 1.15.7
* Remove 1.16 and 1.17 conformance tests
* Remove 1.16 to 1.17 upgrade tests

1.17 to 1.18 upgrade tests were intentionally left in place because we
still support upgrading to a supported Kubernetes version.

Signed-off-by: Marko Mudrinić <mudrinic.mare@gmail.com>

* Update containerd E2E tests to a newer K8s version

Signed-off-by: Marko Mudrinić <mudrinic.mare@gmail.com>

* Update Kubernetes versions used in tests

Signed-off-by: Marko Mudrinić <mudrinic.mare@gmail.com>