Remove old CI versions of Ubuntu, Windows and MacOS

Author: fredmorcos

.github/workflows/ci.yml

@@ -18,12 +18,8 @@ jobs:
       matrix:
         os:
           - ubuntu-latest
-          - ubuntu-18.04
-          - ubuntu-16.04
           - windows-latest
-          - windows-2019
           - macos-latest
-          - macos-10.15
         toolchain: [stable]
     steps:
       - uses: actions/checkout@v2