Add downgrade action

[devmotion]

Runs tests with the oldest compatible versions of the dependencies using https://github.com/julia-actions/julia-downgrade-compat

I imagine this could be useful for testing #55 with old versions of PrettyTables.

Edit: Marked as draft since ReadStatTables doesn't even compile with the oldest declared-to-be-compatible versions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (e179392) to head (6cc5ef0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #57   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          11       11           
  Lines        1249     1249           
=======================================
  Hits         1248     1248           
  Misses          1        1           

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