springboot Upgrade to MongoDB 4.0.0-beta1

Upgrade to Spring Data MongoDB 3.0 (Neumann). The update requires MongoDB Driver 4.0 (beta1 at this time).

Switch from com.mongodb.MongoClient and com.mongodb.MongoClientOptions to com.mongodb.client.MongoClient & com.mongodb.MongoClientSettings and share code paths for MongoClientSettings between reactive and imperative configuration.

Closes: #19356

Comment From: snicoll

This broke a smoke test as it looks like we don't connect to the embedded broker properly anymore