update job to run on latest ubuntu

Author: wing328

.github/workflows/ci.yaml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   test:
     # virtual environments: https://github.com/actions/virtual-environments
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
       # Caches and restores the bazelisk download directory.