CompatHelper: bump compat for Flux to 0.15, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the Flux package from 0.12, 0.13, 0.14 to 0.12, 0.13, 0.14, 0.15.
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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.39%. Comparing base (cecd31b) to head (9b0544c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #137   +/-   ##
=======================================
  Coverage   96.39%   96.39%           
=======================================
  Files          22       22           
  Lines         776      776           
=======================================
  Hits          748      748           
  Misses         28       28           

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