.Net: Graduate MongoDB connector

[markwallace-microsoft]

MongoDB.Driver 3.x GA package is available
Need to upgrade and then we can GA
Date time changes need to be updated

Checklist to be completed when graduating an experimental feature

• Notify PM's and EM's that feature is ready for graduation
• Contact PM for list of sample use cases
• Verify there are sample implementations​ for each of the use cases
• Verify telemetry and logging are complete
• ​Verify API docs are complete and arrange to have them published
• Make appropriate updates to Learn docs​
• Make appropriate updates to Concept samples
• Make appropriate updates to Blog posts
• Verify there are no serious open Issues​​
• Update table in EXPERIMENTS.md
• Remove SKEXP​ flag from the experimental code