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-core 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)
• As pointed out in #70, the cs-multibase library recommended by IPFS only supports netstandard1.6. This is a complex thing to create and is becoming an increasingly important part of the spec, so we're bringing this dependency along.
• All .NET implementations (core, framework) that support anything under 2.0 are EOL by now.

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

[ATikadze]

👋