Revert "Update codecov/codecov-action action to v4"

[hugovk]

Reverts #15

v4 is a breaking change requiring setting a token due to rate limiting:

• https://about.codecov.io/blog/january-product-update-updating-the-codecov-ci-uploaders-to-the-codecov-cli/
• Tokenless support in the new CLI (for public repos and forks) codecov/feedback#112 (comment)
• • Tokenless support in the new CLI (for public repos and forks) codecov/feedback#112 (comment)

Let's revert and stick with v3 for now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4816978) 100.00% compared to head (5f11db0) 100.00%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #16   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           51        51           
=========================================
  Hits            51        51           

Flag	Coverage Δ
macos-latest	100.00% <0.00%> (ø)
ubuntu-latest	100.00% <0.00%> (ø)
windows-latest	100.00% <0.00%> (ø)

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