Update christophwurst/nextcloud requirement from ^17.0 to ^18.0

Updates the requirements on [christophwurst/nextcloud](https://github.com/ChristophWurst/nextcloud_composer) to permit the latest version.
- [Release notes](https://github.com/ChristophWurst/nextcloud_composer/releases)
- [Commits](https://github.com/ChristophWurst/nextcloud_composer/compare/v17.0.0...v18.0.0)

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

composer.json

@@ -24,7 +24,7 @@
 		}
 	},
 	"require-dev": {
-		"christophwurst/nextcloud": "^17.0",
+		"christophwurst/nextcloud": "^18.0",
 		"phpunit/phpunit": "^8.2",
 		"league/factory-muffin": "^3.0",
 		"league/factory-muffin-faker": "^2.0"