make CLI errror indicate the limitation of non-GA API suppoer

[errordeveloper]

Per @stefanprodan's comment in in #4490 (comment):

Yes, the CLI ensures backwards compatibility only for GA APIs, for beta versions you need a CLI that matches the cluster version.

The error message is currently this, which is a bit confusing:

✗ failed to get API group resources: unable to retrieve the complete list of server APIs: helm.toolkit.fluxcd.io/v2beta2: the server could not find the requested resource

It'd be good if the error message was more explicit about the limitations of non-GA APIs in the CLI.