net standard 2.1 update

[xixaoly]

• lib target net45 to net standard 2.1 update
• tests target net 5 update
• RestSharp calls update

[michaldivis]

I'd like to support this PR. This is a much needed change.

[michaldivis]

This fixes my issue #15, please merge...

[michaldivis]

please merge this

[xpetrik]

It looks like @FSichinger died. Thank you @xixaoly for your work. We will use your PR and we will include source filles directly without nuget package.

[FSichinger]

Thanks for pull request and I apologize for late reply.

Next release will be available soon

[jiripolasek]

Hello,
is it necessary to upgrade all the way to .NET Standard 2.1 only? It limits the possible audience to .NET Core/.NET only as there is no .NET Framework that supports .NET Standard 2.1.
PR: #19

[xpetrik]

This is correct and necessary way to target both frameworks. @FSichinger will surely confirm this PR before he release nuget package

[jiripolasek]

The issue here is the version of .NET Standard used: targeting .NET Standard 2.0 (instead of 2.1) enables support for .NET Framework 4.6.1 - 4.8. Otherwise .NET Frameworks will be cut off.