Update drivers ecosystem page with new C# driver version

Description

Please update the following page:

https://docs.mongodb.com/ecosystem/drivers/csharp/

To reflect that 2.8.1 is the current version of the C# driver.

The MongoDB Compatibility and .NET Language Compatibility matrix entries for 2.8 should be the same as those for 2.7.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)