.NET Standard 2.0 support

[mngobbi]

Why?

• At the time, current nuget package (2.1.1) does not support netstandard installations.
• Tests projects are targeting against "netcore2.0" which already hit EOL.
• Adding support to full framework 4.5.2 on https://github.com/intercom/intercom-dotnet/pull/147 has broken CircleCI builds
• full framework 4.5.2 has lost official support on past April 26
• If we want to add full netstandard2.0 support, nugets must be updated, making 4.5.2 support really difficult

How?

• Updated netcore2.0 to netcore3.1 (LTS support) on tests projects
• Removed net452 as a second target framework in the .csproj files
• Updated referenced NuGet packages
• Fixed CircleCI builds (using new docker image)

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.NET standard 2.0 supports:

• .NET Core 2.0
• .NET Framework 4.6.1
• Mono 5.4
• Xamarin.iOS 10.14
• Xamarin.Mac 3.8
• Xamarin.Android 8.0
• Universal Windows Platform 10.0.16299