CompatHelper: bump compat for MLJFlux to 0.4, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the MLJFlux package from 0.2, 0.3 to 0.2, 0.3, 0.4.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.39%. Comparing base (835e389) to head (f1b8d1a).
Report is 22 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #19   +/-   ##
=======================================
  Coverage   63.39%   63.39%           
=======================================
  Files           7        7           
  Lines         265      265           
=======================================
  Hits          168      168           
  Misses         97       97           

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