Add option to upgrade only control-plane nodes in talosctl upgrade-k8s command

[donch]

Feature Request

Add an option to the talosctl upgrade-k8s command to upgrade only the control-plane nodes, leaving worker nodes at their current version. This would allow administrators to perform successive control-plane upgrades before upgrading worker nodes only once, reducing the overall impact on the cluster and providing more flexibility in the upgrade process.

Description

Currently, the talosctl upgrade-k8s command upgrades all nodes in the cluster. However, according to the official Kubernetes version skew policy (https://kubernetes.io/releases/version-skew-policy/#kubelet), kubelet can be up to three minor versions behind the API server. This allows for a more flexible and less impactful upgrade strategy, especially for large clusters.

Proposed implementation:

Add a new flag to the upgrade-k8s command, such as --control-plane-only or -c, which when specified would limit the upgrade process to control-plane nodes only.