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 #345

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7003b61) to head (7482765).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #353   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          111       111           
  Lines         1082      1082           
  Branches       159       159           
=========================================
  Hits          1082      1082           

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[pomegranited]

👍 I'm not a CC here, but will see whether my approval moves this along :)

• I tested this green CI and npm publishing is sufficient (which we'll see when this merges)
• I read through the code and checked for breaking changes.