Update phpunit/phpunit requirement from ^8.0 to ^8.0 || ^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-9.2.md)
- [Commits](sebastianbergmann/phpunit@8.0.0...9.2.6)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

composer.json

@@ -19,7 +19,7 @@
     "require-dev": {
         "phpro/grumphp": "^0.17.1",
         "phpstan/phpstan": "^0.11.5",
-        "phpunit/phpunit": "^8.0",
+        "phpunit/phpunit": "^8.0 || ^9.0",
         "squizlabs/php_codesniffer": "^3.4",
         "symfony/var-dumper": "^4.2 || ^5.0"
     },