Downgrade codecov-action from v5 to v4

[thijsjanzen]

For some reason, it only works with v4.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.88%. Comparing base (814dca9) to head (391b63b).
⚠️ Report is 170 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   88.73%   86.88%   -1.85%     
==========================================
  Files          14       18       +4     
  Lines        2059     2646     +587     
==========================================
+ Hits         1827     2299     +472     
- Misses        232      347     +115     

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