GitHub action CI: drop ubuntu-16.04 (#1254)

Since ubuntu-16.04 is rather dated, drop it in GitHub action CI.

.github/workflows/ci.yml

@@ -12,7 +12,6 @@ jobs:
     strategy:
       matrix:
         os:
-        - ubuntu-16.04
         - ubuntu-18.04
         - ubuntu-20.04
       fail-fast: false