Remove Ubuntu 20 no longer supported by actions.

Author: wb2osz

.github/workflows/ci.yml

@@ -71,15 +71,6 @@ jobs:
               build_type: 'Release',
               cmake_extra_flags: ''
             }
-          - {
-              name: 'Ubuntu 20.04',
-              os: ubuntu-20.04,
-              cc: 'gcc',
-              cxx: 'g++',
-              arch: 'x86_64',
-              build_type: 'Release',
-              cmake_extra_flags: ''
-            }
 
     steps:
       - name: checkout