github workflow: stop testing ubuntu-20.04

Because of forthcoming removing of runners.

Author: eserte

.github/workflows/test.yml

@@ -16,7 +16,6 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: 'ubuntu-20.04'
           - os: 'ubuntu-22.04'
           - os: 'ubuntu-24.04'