Deletes ubuntu-16.04 from CI b/c it's deprecated

For more details, see: actions/runner-images#3287

Author: NeedleInAJayStack

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [macos-10.15, macos-latest, ubuntu-16.04, ubuntu-18.04, ubuntu-20.04]
+        os: [macos-10.15, macos-latest, ubuntu-18.04, ubuntu-20.04]
     steps:
     - uses: actions/checkout@v2
     - name: Set code coverage path