Ubuntu 16.04 is no longer supported on GitHub Actions

Author: eregon

.github/workflows/build.yml

@@ -48,7 +48,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-16.04, ubuntu-18.04, ubuntu-20.04, macos-latest ]
+        os: [ ubuntu-18.04, ubuntu-20.04, macos-latest ]
         name: [ head, debug ]
     runs-on: ${{ matrix.os }}
     steps: