build: Switch to ubuntu-latest for builds

This code does not have any dependencies that are specific to any specific
version of ubuntu.  So instead of testing on a specific version and then needing
to do work to keep the versions up-to-date, we switch to the ubuntu-latest
target which should be sufficient for testing purposes.

This work is being done as a part of openedx/platform-roadmap#377

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       matrix:
         os:
-          - ubuntu-20.04
+          - ubuntu-latest
         python-version:
           - 3.8
         toxenv: [ formatting,quality,python ]

.github/workflows/pypi-publish.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   push:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
       - name: Checkout