ci: Change to ubuntu 16 cores

[mzaniolo]

We need to go back to ubuntu 16 cores because the build of the docker image is failing on the backend system test. See issue #36

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.44%. Comparing base (88e7e7f) to head (4c9347d).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #37   +/-   ##
=======================================
  Coverage   44.44%   44.44%           
=======================================
  Files           1        1           
  Lines           9        9           
=======================================
  Hits            4        4           
  Misses          5        5           

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