Hide the Virtual Clusters explorer product in the local cluster's cluster explorer

[mantis-toboggan-md]

We should not include a 'virtual clusters' nav item in the local cluster's cluster explorer. This nav item features an easy way to install k3k, and we should discourage installing k3k in the local cluster.

[mantis-toboggan-md]

We can't actually do this yet, my b- rancher/dashboard#15842

for the time being, we'll update the landing page to hide the "Install k3k" button and warn users against doing that in the local cluster