refactor: reintroduce the .NET Framework 4.8 target framework

[Dor-bl]

List of changes

I've been contacted by numerous people saying the fact we dropped the support for .NET Framework 4.8 has blocked them from upgrading to latest RC versions With this PR we should be able to have backward compatability with the Net48

Types of changes

What types of changes are you proposing/introducing to the .NET client? Put an x in the boxes that apply

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] New feature (non-breaking change which adds value to the project)
• [ ] Breaking change (fix or feature that would cause existing functionality not to work as expected)

Documentation

• [ ] Have you proposed a file change/ PR with appium to update documentation?

  This can be done by navigating to the documentation section on http://appium.io selecting the appropriate command/endpoint and clicking the 'Edit this doc' link to update the C# example

Integration tests

• [ ] Have you provided integration tests to pass against the beta version of appium? (for Bugfix or New feature)

Details

Please provide more details about changes if it is necessary. If there are new features you can provide code samples showing how they work and possible use cases. Also, you can create gists with pasted C# code samples or put them here using markdown. About markdown please read Mastering markdown and Writing on GitHub

[Dor-bl]

@MartyIX your feedback would be great if possible.

[MartyIX]

In general, code looks OK to me. But I have not tested the PR.