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

closes #478

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04]
+        os: [ubuntu-latest]
         python-version:
         - '3.8'
         - '3.11'
@@ -51,7 +51,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04]
+        os: [ubuntu-latest]
         python-version:
         - '3.8'
         - '3.11'