Remove ubuntu-20.04 from CI

See actions/runner-images#11101

Author: tianon

.github/workflows/ci.yml

@@ -17,7 +17,6 @@ jobs:
         os:
           - ubuntu-24.04
           - ubuntu-22.04
-          - ubuntu-20.04
           - windows-2025
           - windows-2022
           - windows-2019