chore(CI): Adjust testing matrix for Nextcloud 34 on stable34

Signed-off-by: Joas Schilling <coding@schilljs.com>

Author: nickvergessen

composer.json

@@ -28,7 +28,7 @@
 		"test:unit": "vendor/bin/phpunit -c tests/phpunit.xml"
 	},
 	"require-dev": {
-		"nextcloud/ocp": "dev-master",
+		"nextcloud/ocp": "dev-stable34",
 		"bamarni/composer-bin-plugin": "^1.8"
 	},
 	"autoload-dev": {