Build on Ubuntu 20.04 as 18.04 is deprecated

Author: cknitt

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
       matrix:
         os:
           - macos-latest
-          - ubuntu-18.04
+          - ubuntu-20.04
           - windows-latest
 
     runs-on: ${{ matrix.os }}