Deprecate older ILogger implementation in package and move to newer implementation #817

A newer implementation of the ILogger is now pulled into a separate package in PR microsoft/ApplicationInsights-dotnet-logging#239

The current implementation in ASPNetCore package needs to be deprecated and guidance needs to be updated to ask people to move to the newer implementation.

• Mark current ILogger implementation in this repo as deprecated
• Decide if ASPNetCore package takes a dependency on the new ILogger package or we directly ask developers to install both.
• Based on decision above we might need to mark the existing extension method in ASPNetCore package as deprecated.