Reconsider default values in cluster charts

[yulianedyalkova]

With the new releases flow, a lot of the values in our cluster chart get overwritten by the values in the release file. Let's either remove those or make it explicit as currently it's a bit confusing (thread).

Initial scope:

• Clearly document which values get overwritten and not used anymore for both CAPA and CAPZ

[nprokopic]

Different providers still have different levels of integration with cluster chart, and not all of them still use new releases, so eventhough some default values are not used any more by e.g. cluster-aws / CAPA, they could be still used by other providers.

e.g. The default k8s version in the schema is used by providers that use cluster chart, but do not use new releases. Until last week that was still CAPZ (which is still on 1.25), and I think CAPZ still needs that, since v25 with new releases is not yet merged giantswarm/releases#1296.

We also have a helper in cluster chart for getting a Kubernetes version https://github.com/giantswarm/cluster/blob/6017f6ed54d4152df6df94d5dbf04dae2c72e502/helm/cluster/templates/_helpers.tpl#L368. We are already using this in cluster-aws and now/soon in cluster-azure. This helper func will get k8s version form either Release CR or from Helm values, depending on if the provider is using new releases or not. Unfortunately this (and some other similar "public" helpers) is not yet documented in some more visible place. We can do this and maybe have some docs about these in cluster chart README.