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

@@ -14,7 +14,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04]
+        os: [ubuntu-latest]
         python-version: ['3.8', '3.11', '3.12']
         toxenv: ['django42']
 

.github/workflows/publish_pypi.yml

@@ -8,7 +8,7 @@ on:
 jobs:
 
    push:
-     runs-on: ubuntu-20.04
+     runs-on: ubuntu-latest
 
      steps:
        - name: Checkout