Keep phpstan/phpstan-phpunit in sync with the forced phpunit/phpunit upgrade on lowest-deps CI

[sylfabre]

Summary

• The lowest-deps CI job force-upgrades phpunit/phpunit (and a few other tools) to their latest version after composer-install resolves lowest bounds, but left phpstan/phpstan-phpunit behind at whatever old version it resolved to.
• Once a project's test suite uses PHPUnit's ->expects()->method() mock chain, the stale phpstan-phpunit extension references a PHPUnit mock class that no longer exists in the force-upgraded PHPUnit, and vendor/bin/phpstan fails with a class.notFound error on the lowest matrix legs.
• Confirmed and fixed the same issue in assoconnect/validator-bundle#85; porting the fix here since this template ships the same workflow and the same assoconnect/php-quality-config dependency that pulls in phpstan/phpstan-phpunit.

Test plan

• Reproduced locally in Docker (php:8.4-cli + bcmath/intl): composer update --prefer-lowest --prefer-stable then the tool-upgrade step reproduces phpstan/phpstan-phpunit staying at 2.0.0 while phpunit/phpunit jumps to 13.x, triggering the class.notFound error on a test using ->expects()->method().
• With phpstan/phpstan-phpunit added to the upgrade step, phpstan/phpstan-phpunit upgrades to 2.0.18 alongside phpunit/phpunit and the error disappears.

🤖 Generated with Claude Code