Don't run tests against ubuntu 20.04 which was deprecated from GitHub workflows in early 2025

This is a scheduled Ubuntu 20.04 retirement. Ubuntu 20.04 LTS runner will be removed on 2025-04-15. For more details, see actions/runner-images#11101

Author: andreineculau

.github/workflows/run-bats-core-tests.yml

@@ -29,7 +29,8 @@ jobs:
       matrix:
         os:
           [
-            ubuntu-20.04,
+            # ubuntu 20.04 runner was deprecated early 2025
+            # ubuntu-20.04,
             ubuntu-22.04,
             ubuntu-24.04,
             ubuntu-latest,