Update phpunit/phpunit requirement from ^7 to ^8

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-8.5.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/compare/7.0.0...8.5.8)

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

composer.json

@@ -15,6 +15,6 @@
 	"require-dev": {
         "christophwurst/nextcloud": "^19.0",
 		"nextcloud/coding-standard": "^0.3.0",
-		"phpunit/phpunit": "^7"
+		"phpunit/phpunit": "^8"
 	}
 }