Azure: Move to windows-2019

As per actions/runner-images#4312 the
Windows-2016 environments are scheduled for deprecation and removal in
early 2022.  Move to windows-2019 now to avoid this (Visual Studio 2019
is included here, hence the tag naming scheme change).

Signed-off-by: Tom Rini <trini@konsulko.com>

.azure-pipelines.yml

@@ -1,5 +1,5 @@
 variables:
-  windows_vm: vs2017-win2016
+  windows_vm: windows-2019
   ubuntu_vm: ubuntu-18.04
   macos_vm: macOS-10.15
   ci_runner_image: trini/u-boot-gitlab-ci-runner:focal-20210921-05Oct2021