Drop use of deprecated ubuntu-20 runner

Author: jim-easterbrook

.github/workflows/test-interface.yml

@@ -31,8 +31,6 @@ jobs:
         python-version: ['3.8', '3.9', '3.10', '3.11', '3.12']
         os: [ubuntu-latest]
         include:
-          - python-version: '3.6'
-            os: ubuntu-20.04
           - python-version: '3.7'
             os: ubuntu-22.04
     runs-on: ${{ matrix.os }}