Update link to k8s-status to use toolforge.org domain

Summary:
*.toolforge.org is current canonical Toolforge tool domain.
Change the link to k8s-status to use it -- the tool itself
appears to work fine.

This is to avoid an unnecessary redirect.

Test Plan:
Verify the "Kubernetes cluster status" link
at admin.toolforge.org points to k8s-status.toolforge.org,
not tools.wmflabs.org/k8s-status.

Reviewers: bd808, Majavah

Tags: Tool-admin, User-Urbanecm

Differential Revision: https://phabricator.wikimedia.org/D1197