Update runners to ubuntu-24.04 from deprecated ubuntu-20.04 label (googleapis#26480)

Co-authored-by: Bill Napier <napier@google.com>

Author: google-admin

.github/workflows/update-root-readme.yaml

@@ -7,7 +7,7 @@ on:
 name: update root README.md
 jobs:
   update:
-    runs-on: ubuntu-20.04
+    runs-on: 'ubuntu-24.04'
     steps:
       - uses: actions/checkout@v2
         with: