Drop ubuntu-16.04 CI target. (#169)

Author: yossigo

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
   build-ubuntu:
     strategy:
       matrix:
-        platform: [ubuntu-latest,ubuntu-16.04]
+        platform: [ubuntu-latest, ubuntu-18.04]
     runs-on: ${{ matrix.platform }}
     steps:
     - uses: actions/checkout@v2