phpunit/phpunit should be in composer's require section, not require-dev

[chrif]

I'm trying to upgrade PHPUnit from 9 to 10 in a project where we are using blackfire/php-sdk:2.5.7 in our test suite. Composer did not complain, but PHPStan found an unknown class after the update:

Internal error: Interface "PHPUnit\Runner\BeforeFirstTestHook" not found while analysing file vendor/blackfire/php-sdk/src/Blackfire/Bridge/PhpUnit/BlackfireBuildExtension.php

I should have gotten a composer's constraint conflict instead when upgrading. I think phpunit/phpunit should not be a dev dependency only for blackfire/php-sdk.