Bump phpunit/phpunit from 8.5.31 to 9.6.18

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 8.5.31 to 9.6.18.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.18/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@8.5.31...9.6.18)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

composer.json

@@ -24,7 +24,7 @@
         "php": ">=7.4"
     },
     "require-dev": {
-        "phpunit/phpunit": "^8",
+        "phpunit/phpunit": "^8 || ^9",
         "ext-dom": "*"
     }
 }