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

Author: feanil

.github/workflows/ci.yml

@@ -14,8 +14,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os:
-        - ubuntu-20.04
+        os: ["ubuntu-latest"]
         python-version: ['3.8', '3.11']
         toxenv: [django42]
 

.github/workflows/pypi-release.yml

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