ci: update Windows runner to windows-2019

The windows-2016 runner will soon be removed:
https://github.blog/changelog/2021-10-19-github-actions-the-windows-2016-runner-image-will-be-removed-from-github-hosted-runners-on-march-15-2022/
Therefore, switch to the next version.

.github/workflows/windows.yml

@@ -9,7 +9,7 @@ on:
 
 jobs:
   build-windows:
-    runs-on: windows-2016
+    runs-on: windows-2019
     steps:
       - name: Install Go
         uses: actions/setup-go@v2