remove Windows 2016 from tests.

see actions/runner-images#5238

Author: shirou

.github/workflows/test.yml

@@ -5,7 +5,7 @@ jobs:
     strategy:
       matrix:
         go-version: [1.16.x, 1.17.x]
-        os: [ubuntu-20.04, ubuntu-18.04, windows-2019, windows-2016, macOS-10.15, macos-11]
+        os: [ubuntu-20.04, ubuntu-18.04, windows-2019, macOS-10.15, macos-11]
     runs-on: ${{ matrix.os }}
     steps:
     - name: Install Go