docs: fix migration guide URL for v4

[claudiobusatto]

📋 Changes

This PR fixes the broken link to the V4 migration guide in CHANGELOG.md.
Previously, the link pointed to a non-existent page on GitHub.

🎯 Testing

Before

1. Open actual CHANGELOG.md on the main branch.
2. Navigate to v4.0.0 section.
3. Click on MIGRATION GUIDE link, it will redirect to a 404 page.

After

1. Open CHANGELOG.md from this PR.
2. Navigate to v4.0.0 section.
3. Click on MIGRATION GUIDE link. It will redirect to the correct documentation page.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.61%. Comparing base (0ed21d1) to head (dd073f0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2129   +/-   ##
=======================================
  Coverage   82.61%   82.61%           
=======================================
  Files          21       21           
  Lines        2042     2042           
  Branches      358      358           
=======================================
  Hits         1687     1687           
  Misses        348      348           
  Partials        7        7           

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