minor #268 prevent PHPUnit 10 from being installed (xabbuh)

This PR was merged into the 2.x branch.

Discussion
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prevent PHPUnit 10 from being installed

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets |
| License       | MIT

Since 5.2 the matthiasnoback/symfony-config-test package as a requirement for phpunit/phpunit. As we are using PHPUnit 9.6 to run tests we need to make sure that we do not get conflicting versions of the same package.

Commits
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7693564 prevent PHPUnit 10 from being installed

Author: xabbuh

composer.json

@@ -41,6 +41,7 @@
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^3.41",
         "matthiasnoback/symfony-dependency-injection-test": "^4.0 || ^5.0",
+        "phpunit/phpunit": "^9.6",
         "symfony/console": "^5.4 || ^6.0 || ^7.0",
         "symfony/phpunit-bridge": "^5.4 || ^6.0 || ^7.0",
         "symfony/yaml": "^5.4 || ^6.0 || ^7.0"