sandbox: change from n1 to e2 instances
sandbox: change from n1 to e2 instances

This conforms with a policy change, and should save money with no
measurable performance impact.

Removes bradfitz's SSH key from instances (that are inaccessible
anyway).

Updates golang/go#25224

Change-Id: I1733192c98deee1deabf2237ae5fe19edd29ab93
Reviewed-on: https://go-review.googlesource.com/c/playground/+/229957
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/playground,golang/playground,golang/playground