chore: Move phpunit to vendor-bin and update to 9.6

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

Author: come-nc

composer.json

@@ -28,7 +28,6 @@
 		"test:unit": "vendor/bin/phpunit -c tests/phpunit.xml"
 	},
 	"require-dev": {
-		"phpunit/phpunit": "^9.5",
 		"nextcloud/ocp": "dev-master",
 		"bamarni/composer-bin-plugin": "^1.8"
 	},