Update phpunit/phpunit requirement from ^8.0 to ^8.0 || ^9.0 (#22)

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/9.5.28/ChangeLog-9.5.md)
- [Commits](sebastianbergmann/phpunit@8.0.0...9.5.28)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

composer.json

@@ -28,7 +28,7 @@
         "smarty-plugins"
     ],
     "require-dev": {
-        "phpunit/phpunit": "^8.0",
+        "phpunit/phpunit": "^8.0 || ^9.0",
         "symfony/cache": "^5.0|^6.0",
         "bentools/cartesian-product": "^1.3",
         "symfony/polyfill-php80": "^1.27",