Update phpunit/phpunit requirement from ^5.4 to ^8.2

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-8.2.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/compare/5.4.0...8.2.1)

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

composer.json

@@ -15,7 +15,7 @@
 	],
 	"require-dev": {
 		"christophwurst/nextcloud": "^15.0",
-		"phpunit/phpunit": "^5.4",
+		"phpunit/phpunit": "^8.2",
 		"league/factory-muffin": "^3.0",
 		"league/factory-muffin-faker": "^2.0"
 	}