gen: run on ubuntu-latest

ubuntu-20.04 is being deprecated.

See https://github.com/actions/runner-images/issues/11101.

.github/workflows/gen.yml

@@ -12,7 +12,7 @@ on:
 
 jobs:
   codegen:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v4