Remove Python 3.8 workflow

Author: awaelchli

.github/workflows/cpu-tests.yml

@@ -28,7 +28,6 @@ jobs:
           - {os: "ubuntu-22.04", python-version: "3.11"}
           - {os: "ubuntu-22.04", python-version: "3.10"}
           - {os: "ubuntu-22.04", python-version: "3.9"}
-          - {os: "ubuntu-20.04", python-version: "3.8"}
           - {os: "windows-2022", python-version: "3.10"}
     timeout-minutes: 25