Inform people to set environment namespace instead of KUBE_NAMESPACE CI variable

This relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/67273 where we do not want

When we have https://gitlab.com/gitlab-org/gitlab-ce/issues/59638 there should be no reason for anyone to want to set the KUBE_NAMESPACE CI variable when using our Kubernetes integration.

We should display a warning under the variable on the variables page if anyone does set this and this warning should point them to configure this for the environment instead.

One possible downside here is that the user still cannot accomplish this for wildcards like they used to be able to with the CI variable but then the CI variable never worked properly anyway so we should still discourage it.