Update christophwurst/nextcloud requirement from ^19.0 to ^20.0

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/v19.0.0...v20.0.0)

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

composer.json

@@ -13,7 +13,7 @@
 		"lint": "find . -name \\*.php -not -path './vendor/*' -print0 | xargs -0 -n1 php -l"
 	},
 	"require-dev": {
-        "christophwurst/nextcloud": "^19.0",
+        "christophwurst/nextcloud": "^20.0",
 		"nextcloud/coding-standard": "^0.3.0",
 		"phpunit/phpunit": "^8"
 	}