Update the mentions of testgrid dashboard URL from k8s-testgrid.appspot.com to testgrid.k8s.io

[dhruvmehtaaa]

What happened:

Somewhere down the line, the domain name of the testgrid dashboard was updated from k8s-testgrid.appspot.com to testgrid.k8s.io. After the update old domain now throws a 'page not found error'. There're numerous links to the old testgrid dashboard's domain which across many projects now stands broken.

What you expected to happen:
This issue aims to track the update of the testgrid dashboard's URL across different projects. Once updated we expect to have no broken links to the testgrid dashboard.

How to reproduce it (as minimally and precisely as possible):
Visit: https://k8s-testgrid.appspot.com/

[dhruvmehtaaa]

/assign