Email alerts should use the k8s.testgrid.io instead of k8s-testgrid.appspot.com/

## Observed behavior
Email alerts from TestGrid use the hostname `k8s-testgrid.appspot.com` instead of `k8s.testgrid.io`

## Desired behavior
For the alerts can we move to use `k8s.testgrid.io` please?

## Rationale
This will make process automation easier by reducing the amount of code that needs to be written to handle two host names.

## Sample alert
Text copied and pasted from gmail. I had to manually format this in this issue but it's just to show the appsot links in the message.

> From: noreply+testgrid via release-team  26 Nov 2020, 23:25 (2 days ago)
> Subject : New TestGrid alerts at sig-release-master-blocking#kind-ipv6-master-parallel since commit/5ed4b76a0.
>
> **New Test Failures in kind-ipv6-master-parallel.**
> Test [Overall](https://k8s-testgrid.appspot.com/sig-release-master-blocking#kind-ipv6-master-parallel) has failed 3 times . It passed and failed at commit/5ed4b76a0. Earliest failure found at commit/5ed4b76a0 (Thu Nov 26 12:43 PST).
>
>
>
> TestGrid Link: https://k8s-testgrid.appspot.com/sig-release-master-blocking#kind-ipv6-master-parallel
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