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 82.66%. Comparing base (3e5fa12) to head (a04f62c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2129   +/-   ##
=======================================
  Coverage   82.66%   82.66%           
=======================================
  Files          21       21           
  Lines        2060     2060           
  Branches      362      362           
=======================================
  Hits         1703     1703           
  Misses        350      350           
  Partials        7        7           

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[arpit-jn]

Thanks for this contribution, @claudiobusatto. To get this merged, could you please sign your commit?

If you're not able to, no problem. Just let us know, and we'll cherry-pick your changes into a new PR to get this merged before the end of the week.