chore: migrate coverage badge to codecov

[ohmyfelix]

What changed

• replaced the README coverage badge link and image with the Codecov variant used in other migrated Contributte packages
• kept the existing reusable coverage workflow in place because this repository already uses the Codecov-ready nette-tester-coverage-v2 workflow

Why

• finishes the remaining Coveralls-to-Codecov migration for contributte/apitte
• aligns this package with Migrate remaining Coveralls usage to Codecov across contributte repos contributte#72 without duplicating repository coverage wiring changes that are already done

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.12%. Comparing base (33ec0d4) to head (c8b89e3).
⚠️ Report is 23 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
+ Coverage   90.55%   91.12%   +0.57%     
==========================================
  Files         105      102       -3     
  Lines        2413     2174     -239     
==========================================
- Hits         2185     1981     -204     
+ Misses        228      193      -35     

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