github workflow: stop testing ubuntu-20.04

Because of forthcoming removing of runners.

Author: eserte

.github/workflows/test.yml

@@ -17,7 +17,6 @@ jobs:
     strategy:
      matrix:
        include:
-         - os: 'ubuntu-20.04'
          - os: 'ubuntu-22.04'
          - os: 'ubuntu-24.04'
          - os: 'windows-latest'