chore: upgrade CI runner to Ubuntu 20.04 (#12563)

This should fix a broken CI error where apt fails to update:

  E: The repository 'https://download.opensuse.org/repositories/devel:/kubic:/libcontainers:/stable/xUbuntu_18.04  Release' no longer has a Release file.

.github/workflows/superset-docs.yml

@@ -11,7 +11,7 @@ on:
 jobs:
   docs:
     name: build
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout code
         uses: actions/checkout@v2