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

[github-actions]

This pull request changes the compat entry for the Functors package from 0.3 to 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

Base: 86.86% // Head: 86.86% // No change to project coverage 👍

Coverage data is based on head (5c4792f) compared to base (b015b7a).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2128   +/-   ##
=======================================
  Coverage   86.86%   86.86%           
=======================================
  Files          19       19           
  Lines        1469     1469           
=======================================
  Hits         1276     1276           
  Misses        193      193           

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[mcabbott]

Should run tests after FluxML/Optimisers.jl#121