Prevent flux install overriding a bootstrapped cluster

[stefanprodan]

To prevent people from upgrading by mistake a cluster with flux install when the cluster has been bootstrapped, we can look for the kustomize.toolkit.fluxcd.io/name and kustomize.toolkit.fluxcd.io/namespace labels on the gitrepositories.source.toolkit.fluxcd.io CRD. If the CRD exists and has these labels, we should return an error like so:

This cluster has been bootstrapped with Flux <VERSION>! Please use 'flux bootstrap' to upgrade.

The Flux version should be extracted from the app.kubernetes.io/version label value.