Update phpunit/phpunit requirement from ^6.0 || ^7.0 to ^6.0 || ^7.0 || ^8.0

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-8.1.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/8.1.3)

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

Author: dependabot-support

composer.json

@@ -37,7 +37,7 @@
     "ext-json": "*",
     "behat/behat": "~3.0",
     "squizlabs/php_codesniffer": "3.*",
-    "phpunit/phpunit": "^6.0 || ^7.0",
+    "phpunit/phpunit": "^6.0 || ^7.0 || ^8.0",
     "yoanm/php-unit-extended": "~1.0"
   }
 }