📖 clarify semantic meaning of the Version fields

[rccrdpccl]

What this PR does / why we need it:
The aim of this PR is to clarify the semantic meaning of the Version fields. At the moment, the documentation heavily implies that version fields should contain a Kubernetes version.
However this is problematic when working with Kubernetes distributions, especially the ones with a versioning schema totally different from Kubernetes (i.e. OpenShift).

UX problem

At the moment, to install a k8s distribution, the user needs to work out what k8s version corresponds to the desired k8s distribution version. However, this is not even always possible: for example, in the OpenShift case, a minor release is based on a given Z patch of k8s, and other Z releases for the same Y version might be based on the same k8s Z patch. As a practical example, OpenShift 4.17.0 is based on 1.30.4 and OpenShift 4.17.1 is also based on 1.30.4, making a bidirectional conversion actually impossible.

Kubernetes version references

Unfortunately this is not straightforward, as there are multiple references to explicit Kubernetes versions.

• machine preflight check explicitly referencing a kubernetes version (since when the skew version policy changes). In this case the referenced version is 1.28.0, which should fall out of support in less than 12 months time
• setting minimum version. In this case, we might be able to drop this check as Cluster API already doesn't support versions below 1.22 (in this case we're toggling between 1.20 and 1.22)

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[fabriziopandini]

I don't mind adding distribution after kubernetes version in go doc, many companies use custom versioning schema, but as a matter of fact the codebase is full of assumptions that this field has a direct correlation with the Kubernetes version being installed.

Top of mind we have similar assumptions in KCP/CABPK, most probably also in the topology controller and the E2E test framework, and I would not be surprised if we have assumption on version in other places as well.

Also, the K8s version being installed is a key information we need to have for implementing support of any new K8s version whenever it introduces a change that requires special handling (and this happens more often that you might expect); also the entire test matrix for our supported versions is based on this information.

This make me wondering if we are going down on a slippery path if we start decoupling the kubernetes distribution version from the actual kubernetes version being installed.

@vincepri @enxebre @sbueringer opinions? should we move this discussion to an issue first?

[fabriziopandini]

Also @chrischdi

[enxebre]

Generally speaking, moving towards having better supportability for distros to handle their versioning makes sense to me. That should hopefully come without any loss or impact for KCP/CABPK/... and for the special handling scenarios, while still enabling other building blocks to replace KCP/CABPK in a meaningful manner. A less convoluted path to satisfy both scenarios might be having a new field for the distro version? so then we have semantics for both and is up to implementers how to leverage them together. I agree with Fabrizio that this deserves thorough discussion.