Upgrade the Mongo internal client and Mongo Chat history to Mongo driver V2

[zamedic]

There are 2 areas of the LangchainGo stack that still use the Mongo V1 driver, namely

1. The Internal Mongo Client
   

   langchaingo/internal/mongodb/client.go

   Line 6 in 509308f

   "go.mongodb.org/mongo-driver/mongo"

2. Mongo Memory
   

   langchaingo/memory/mongo/mongo_chat_history.go

   Line 11 in 509308f

   "go.mongodb.org/mongo-driver/mongo"

I propose to upgrade these to Mongo V2