CI: remove ubuntu-16.04

Author: fridtjof

.github/workflows/main.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-16.04, ubuntu-18.04, ubuntu-20.04]
+        os: [ubuntu-18.04, ubuntu-20.04]
 
     steps:
       - name: Checkout