fix: update PHPUnit minimum version for PHP 7.2 compatibility

- Lower PHPUnit minimum version from ^8.5.40 to ^8.5.21
- This ensures compatibility with PHP 7.2 lowest dependencies
- Addresses GitHub Actions test failure for PHP 7.2 with Symfony 4.4.* lowest dependencies

Author: Litarnus

composer.json

@@ -34,7 +34,7 @@
         "phpstan/phpstan": "1.12.5",
         "phpstan/phpstan-phpunit": "1.4.0",
         "phpstan/phpstan-symfony": "1.4.10",
-        "phpunit/phpunit": "^8.5.40||^9.6.21",
+        "phpunit/phpunit": "^8.5.21||^9.6.21",
         "symfony/browser-kit": "^4.4.20||^5.0.11||^6.0||^7.0",
         "symfony/cache": "^4.4.20||^5.0.11||^6.0||^7.0",
         "symfony/dom-crawler": "^4.4.20||^5.0.11||^6.0||^7.0",