Reduce GitHub Actions OS matrix

.github/workflows/build.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [windows-2016, windows-2019, macos-10.15, ubuntu-16.04, ubuntu-18.04, ubuntu-20.04]
+        os: [windows-2019, macos-10.15, ubuntu-20.04]
     env:
       BUILD_TYPE: Release
     steps:
@@ -58,4 +58,4 @@ jobs:
         prerelease: ${{ !startsWith(github.ref, 'refs/tags/v') || contains(github.ref, '-pre') }}
         files: ${{ github.event.repository.name }}-${{ steps.osname.outputs.lowercase }}.zip
       env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}