Restricting the test pack to 9.5 until symfony/phpunit-bridge supports 10

[weaverryan]

Currently, following the docs:

composer require --dev symfony/test-pack
php bin/phpunit

Fails because this is no longer the correct way to initialize PHPUnit 10: https://github.com/symfony/recipes/blob/b1176e36c42b0738c1475960202d7e074863ca3c/symfony/phpunit-bridge/5.3/bin/phpunit#L11

We could (and should, I think) fix that line to also support PHPUnit 10. But restricting the pack to 9 seems even better, as installing phpunit/phpunit 10 and running vendor/bin/simple-phpunit currently fails, and will fail until symfony/phpunit-bridge supports PHPUnit 10.