Consolidating supported TargetFrameworks to netstandard2.0

[Arlodotexe]

Background

We're forking and maintaining the amazing work done by Richard Schneider, who appears to have permanently logged off since November 2019.

To support IPFS in the .NET ecosystem, I'll be maintaining this repo and the net-ipfs-http-client repo, approving PRs, and generally helping the community pick up where Richard left off.

First steps

Before we can have PRs rolling in, one of the first things we need to do is consolidating our TargetFramework to netstandard2.0.

There's a few reasons for this:

• net45 has since reach end of life
• netstandard1.4 is starting to show its age (also just not recommended anymore)

Checklist

• All TargetFrameworks have been removed, except for netstandard2.0
• All compilation conditionals (#if NET45, etc) have been removed.
• Tests have been consolidated to a single .NET 6 project, referencing a single NS2.0 project

[Arlodotexe]

Linking ipfs-shipyard/net-ipfs-http-client#1