📦 Require phpunit 9 because simple-phpunit is not compatible with phpunit 10 and matthiasnoback/symfony-config-test triggers installation of phpunit 10

From CI
```
PHP Fatal error:  Declaration of PHPUnit\Framework\TestSuite::run(): void must be compatible with PHPUnit\Framework\Test::run(?PHPUnit\Framework\TestResult $result = null): PHPUnit\Framework\TestResult in /home/runner/work/HttplugBundle/HttplugBundle/vendor/phpunit/phpunit/src/Framework/TestSuite.php on line 338
Script vendor/bin/simple-phpunit handling the test event returned with error code 255
```

Author: andrew-demb

composer.json

@@ -57,6 +57,7 @@
         "php-http/cache-plugin": "^1.7",
         "php-http/mock-client": "^1.2",
         "php-http/promise": "^1.0",
+        "phpunit/phpunit": "^9",
         "symfony/browser-kit": "^6.4 || ^7.1",
         "symfony/cache": "^6.4 || ^7.1",
         "symfony/dom-crawler": "^6.4 || ^7.1",