The MongoDbStore is not compatible with the latest versions of mongodb extension. The latest mongodb extension uses a new way of creating the client.

Instead of MongoDB\Client we now have MongoDB\Driver\Manager.

Some options to consider:

1. Make the store work for both interfaces.
2. Migrate the store to the new interface.
3. Create a new MongoDbStore implementation for the latest version of the interface.
4. Use an adapter: https://github.com/alcaeus/mongo-php-adapter.
5. Drop mongodbstore.