dashboard: updates host-windows-amd64-2016 variants

This change updates the image used by the host-windows-amd64-2016
variants. The startup.ps1 was updated with the changes introduced
in CL 503836.

Updates golang/go#52108

Change-Id: I96d5b09309bb33b8c0181e189e6c0c36cad333ce
Reviewed-on: https://go-review.googlesource.com/c/build/+/506895
Run-TryBot: Carlos Amedee <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Auto-Submit: Carlos Amedee <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: cagedmantis

dashboard/builders.go

@@ -587,11 +587,11 @@ var Hosts = map[string]*HostConfig{
 		SSHUsername: "gopher",
 	},
 	"host-windows-amd64-2016": {
-		VMImage:     "windows-amd64-server-2016-v8",
+		VMImage:     "windows-amd64-server-2016-v9",
 		SSHUsername: "gopher",
 	},
 	"host-windows-amd64-2016-big": {
-		VMImage:     "windows-amd64-server-2016-v8",
+		VMImage:     "windows-amd64-server-2016-v9",
 		machineType: "e2-standard-16",
 		SSHUsername: "gopher",
 	},