Update ubuntu-20.04 to ubuntu-24.04

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: imbyc

.github/workflows/gen.yml

@@ -9,7 +9,7 @@ on:
 jobs:
   Generate:
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
 
     steps:
       - uses: actions/checkout@v2