Revert "[BB-2558] Temporarily set default `FORUM_MONGO_AUTH_MECH` to `MONGODB-CR`"
requested to merge revert-626-agrendalath/bb-2558-temporary_backwards-compatible_mongodb_auth into master
Created by: Agrendalath
We upgraded MongoDB authentication method to SCRAM, so we can revert the temporary hack introduced in #626.
Merging this as a trivial PR.