update download-artifact@v3 to download-artifact@v4

[drbenvincent]

There was an error in the GitHub action which uploads after a new version release. This PR updates download-artifact@v3 to download-artifact@v4 given the advice here.

I'll try to manually trigger the release action again after this is merged.

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📚 Documentation preview 📚: https://causalpy--454.org.readthedocs.build/en/454/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.53%. Comparing base (6e19b89) to head (64fe3f6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #454   +/-   ##
=======================================
  Coverage   94.53%   94.53%           
=======================================
  Files          31       31           
  Lines        2068     2068           
=======================================
  Hits         1955     1955           
  Misses        113      113           

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