Remove Deprecated Microsoft.Extensions.PlatformAbstractions

[niemyjski]

This removes .NET Platform 1.0 and 1.1 only supported apis from the core netstandard client.

• https://github.com/aspnet/Announcements/issues/237
• https://learn.microsoft.com/en-us/dotnet/api/microsoft.extensions.platformabstractions.platformservices.default?view=dotnet-plat-ext-1.1
• https://stackoverflow.com/questions/46226283/netstandard-2-0-does-not-have-appdomain-currentdomain-setupinformation-configura
• http://www.hishambinateya.com/goodbye-platform-abstractions
• https://github.com/aspnet/PlatformAbstractions/issues/50

There is a slight change in behavior as I'm trying to reuse some existing logic to smartly detect the entry assembly and exclude Exceptionless and Microsoft Packages (We could use this in a few more spots but wanted to keep this minimal). Here is the diff before using the new GetEntryAssembly helper (which picks a different assembly as our samples start with Exceptionless...)