Downgrade to PHPUnit 9.6

[mnocon]

After the changes from https://github.com/ibexa/version-comparison/pull/132 , the composer install commands fail in this repo.

Example failure:

https://github.com/ibexa/documentation-developer/actions/runs/31193936516/job/92917410245?pr=3345

    Problem 1
      - Root composer.json requires ibexa/version-comparison 5.0.x-dev -> satisfiable by ibexa/version-comparison[5.0.x-dev].
      - Root composer.json requires phpunit/phpunit ^11.0 -> satisfiable by phpunit/phpunit[11.5.50, ..., 11.5.56].
      - phpunit/phpunit[11.5.50, ..., 11.5.56] require sebastian/diff ^6.0.2 -> satisfiable by sebastian/diff[6.0.2].
      - Conclusion: don't install sebastian/diff 6.0.2 (conflict analysis result)

The solution is to downgrade the PHPUnit dependency - from 11 to 9.6 (PHPUnit 10 requires sebastian/diff:^5.0).

This forces us to convert from PHP Attributes to PHPDoc annotations.
With these changes, the tests are running correctly and are passing.

[github-actions]

Preview of modified files: no change to preview.

[github-actions]

Summary

Status	Count
🔍 Total	736713
🔗 Unique	14807
✅ Successful	6220
⏳ Timeouts	0
🔀 Redirected	0
👻 Excluded	730493
❓ Unknown	0
🚫 Errors	0
⛔ Unsupported	0

Full Github Actions output

[mnocon]

Replaced with https://github.com/ibexa/version-comparison/pull/134