Github CI: Ubuntu 20.04 is retired, so try Ubuntu 24.04.

Author: DavidEGrayson

.github/workflows/ci.yaml

@@ -4,7 +4,7 @@ on:
   push:
 jobs:
   ci:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
     - name: Checkout this repository
       uses: actions/checkout@v2