build: Switch to ubuntu-latest for builds

[feanil]

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 #320

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.93%. Comparing base (e630591) to head (d2ffc3f).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #333   +/-   ##
=======================================
  Coverage   67.93%   67.93%           
=======================================
  Files          43       43           
  Lines         789      789           
  Branches      158      158           
=======================================
  Hits          536      536           
  Misses        245      245           
  Partials        8        8           

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