Update phpunit/phpunit requirement from ^4.8.36 || ^7.0 to ^4.8.36 || ^7.0 || ^9.0

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-9.5.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/9.5.4)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

composer.json

@@ -65,7 +65,7 @@
     "require-dev": {
         "ext-zip": "*",
         "ext-gd": "*",
-        "phpunit/phpunit": "^4.8.36 || ^7.0",
+        "phpunit/phpunit": "^4.8.36 || ^7.0 || ^9.0",
         "squizlabs/php_codesniffer": "^2.9 || ^3.5",
         "friendsofphp/php-cs-fixer": "^2.2",
         "phpmd/phpmd": "2.*",