Update codecov-action to v4

[offa]

Fixes the NodeJS 16 deprecation warning:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: codecov/codecov-action@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 33.99%. Comparing base (aaadfa4) to head (412d056).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
- Coverage   34.13%   33.99%   -0.15%     
==========================================
  Files          53       52       -1     
  Lines        2276     2271       -5     
  Branches      337      336       -1     
==========================================
- Hits          777      772       -5     
  Misses       1419     1419              
  Partials       80       80              

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