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 #153

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.78%. Comparing base (f6b9456) to head (d96751b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #161   +/-   ##
=======================================
  Coverage   99.78%   99.78%           
=======================================
  Files          11       11           
  Lines         461      461           
  Branches       33       33           
=======================================
  Hits          460      460           
  Partials        1        1           

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