I installed the official MongoDB app through unraid and it uses Mongo 7 (no option to select a version from the deployment page).
With regards to starting with authentication required, it took me longer than I care to admit, that there are two important environment variables missing that you have to manually add when deploying the docker app:
MONGO_INITDB_ROOT_USERNAME
MONGO_INITDB_ROOT_PASSWORD
Without those, it will not start mongodb with --auth. However, once you add those (and set them) it works as expected, no further modifications needed.