[ClusterClass Operations] Support Kubernetes version update

[salasberryfin]

This is issue is part of the epic #387.

Description

Kubernetes version updates are applied at the Cluster level, which makes cluster management harder especially when dealing with a large number of resources, taking some of the benefits of ClusterClass away from the user. Improving user experience on ClusterClass operations also means simplifying the process of moving to different Kubernetes versions.

Features

The implementation can be very similar to the rebase described in #638, as we can reuse the concept of ClusterTarget to apply changes to one/multiple clusters. It will also be required to control the type of upgrade that is being applied, and whether it is supported or not.

• Let users define one/multiple ClusterTarget and define desired Kubernetes version.
• Control that Cluster objects are listed as target only once (refer to [ClusterClass Operations] Add rebase controller #638 and the validating webhook strategy).
• Avoid unsupported upgrades: control old -> new version.