Target .NET Standard 2.1

[0x414c49]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of Nancy
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

Description

At the moment NancyFX only targets .NET Standard 2.0 and .NET 4.5.2. Since .NET Standard 2.1 is out, we can target the project for .NET Standrd 2.1.

[0x414c49]

In PR #3008, I resolved this issue. The only problem is the +500 analysis warnings for async usages.