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 #87

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.45%. Comparing base (8c883ce) to head (82064dc).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #88   +/-   ##
=======================================
  Coverage   95.45%   95.45%           
=======================================
  Files          16       16           
  Lines        1013     1013           
  Branches       78       78           
=======================================
  Hits          967      967           
  Misses         45       45           
  Partials        1        1           

Flag	Coverage Δ
unittests	95.45% <ø> (ø)

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