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 https://github.com/openedx/platform-roadmap/issues/377

closes https://github.com/openedx/frontend-app-authn/issues/1299

.github/workflows/autoupdate-pull-request.yml

@@ -10,7 +10,7 @@ on:
 jobs:
   autoupdate:
     name: autoupdate
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - uses: docker://chinthakagodawita/autoupdate-action:v1
         env: