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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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