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

closes https://github.com/openedx/i18n-tools/issues/153

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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