Update phpunit/phpunit requirement from ^7.0 to ^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-8.0.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/8.0.1)

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

Author: dependabot-support

composer.json

@@ -19,7 +19,7 @@
         "symfony/css-selector": "^4.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.0",
+        "phpunit/phpunit": "^7.0 || ^8.0",
         "phpdocumentor/phpdocumentor": "^2.9"
     },
     "autoload": {