chore: fix PHPUnit version to ^9.6

[kenjis]

Description

• fix PHPUnit version

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/shield/actions/runs/9068936228/job/24922066021

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