Remove ubuntu-20.04 runner

Author: froggleston

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ "ubuntu-20.04", "ubuntu-22.04", "ubuntu-24.04", "macos-13", "macos-14", "macos-15", "windows-latest" ]
+        os: [ "ubuntu-22.04", "ubuntu-24.04", "macos-13", "macos-14", "macos-15", "windows-latest" ]
         python-version: ["3.9", "3.10", "3.11", "3.12"]
 
     steps: