[ci] windows-2016 image is going away in a bit

https://github.com/actions/virtual-environments/issues/4312

windows-latest is the same as windows-2019, but we are using explicit
windows-2019 image for when they become different.

.github/workflows/msvc-ci.yml

diff --git a/.github/workflows/msvc-ci.yml b/.github/workflows/msvc-ci.yml
index 7c9f5bc..22f22fc 100644
--- a/.github/workflows/msvc-ci.yml
+++ b/.github/workflows/msvc-ci.yml
@@ -12,10 +12,10 @@
 
     strategy:
       matrix:
-        os: [windows-2016, windows-latest]
+        os: [windows-2019, windows-latest]
         include:
-          - name: msvc-2017-x86
-            os: windows-2016
+          - name: msvc-2019-x86
+            os: windows-2019
             ARCH: x86
           - name: msvc-2019-amd64
             os: windows-latest