Don't use retired ubuntu image on CI

Author: garrettjstevens

.github/workflows/python-apollo.yml

@@ -6,7 +6,7 @@ jobs:
   test-python-apollo:
     name: test-python-apollo
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         python-version: [3.9]