[MNT] - Update actions ubuntu image and drop python 3.6 support

[TomDonoghue]

Drops deprecated ubuntu image ('ubuntu-20.04') and correspondingly updates minimum Python version to 3.7 (since 3.6 is no longer supported on the actions tests).

The previously tagged image no longer runs and so needs updated. These leads to dropping Github actions testing for Python v3.6 (unless anyone has any creative suggestions for adding it back). I think it's pretty reasonable to drop official Python 3.6 support from here (it reached end of life by end of 2021 in terms of official Python support). Tagging to ubuntu-22.04 keeps python 3.7 and above.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.21%. Comparing base (771b722) to head (afdfa70).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #156   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files          23       23           
  Lines         919      919           
=======================================
  Hits          875      875           
  Misses         44       44           

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