chore: fix PHPUnit version to ^9.6

[kenjis]

Description

Run vendor/bin/phpunit --verbose --coverage-text --testsuite main
PHPUnit 10.5.20 by Sebastian Bergmann and contributors.

Unknown option "--verbose"
Error: Process completed with exit code 2.

https://github.com/codeigniter4/cache/actions/runs/9207748908/job/25328475291

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [ ] User guide updated
• [ ] Conforms to style guide

[samsonasik]

Thank you @kenjis