lkl: bump up ubuntu image for checkpatch

ubuntu 20.04 runner isn't available recently so, update the version.

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>

Author: thehajime

.github/workflows/ci.yml

@@ -156,7 +156,7 @@ jobs:
           path: /tmp/junit/*.xml
 
   checkpatch:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     name: checkpatch
     steps:
       - name: Checkout