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 #56

.github/workflows/git-tests.yml

@@ -18,7 +18,7 @@ jobs:
           - os: ubuntu-22.04
             git_version: default
 
-          - os: ubuntu-20.04
+          - os: ubuntu-latest
             git_version: default
 
           - os: ubuntu-22.04
@@ -27,7 +27,7 @@ jobs:
           - os: ubuntu-22.04
             git_version: 2.30.2
 
-          - os: ubuntu-20.04
+          - os: ubuntu-latest
             git_version: 2.25.1
 
     steps: