docs(readme): add information for upgrading to v4.x

[FlorianSDV]

INT-696

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.15%. Comparing base (5c291a9) to head (91f1f18).
Report is 11 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #284      +/-   ##
============================================
- Coverage     51.96%   48.15%   -3.81%     
+ Complexity      692      561     -131     
============================================
  Files           152      148       -4     
  Lines          4191     3721     -470     
  Branches         55       55              
============================================
- Hits           2178     1792     -386     
+ Misses         1958     1874      -84     
  Partials         55       55              

Flag	Coverage Δ
backend	76.97% <ø> (-1.07%)	⬇️
frontend	0.00% <ø> (ø)

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[myparcel-bot]

✅ Build successful!

You can find a link to the downloadable artifact below.

Name	Link
Commit	50e8bcf
Logs	https://github.com/myparcelnl/prestashop/actions/runs/11599135807
Download	https://github.com/myparcelnl/prestashop/suites/30300843986/artifacts/2124647595

[myparcel-bot]

🎉 This PR is included in version 4.2.0 🎉

The release is available on GitHub release

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