Notes for .NET 5 / .NET 6

[siegfriedpammer]

Before we are ready to move to .NET 5 / 6, we must change the way plugins are loaded:

• Use AssemblyLoadContext.Default instead of plain Assembly.Load
• Assemblies referenced by plugins need a custom AssemblyLoadContext.Resolving handler

[isaiah]

How much effort is it needed?

[christophwille]

See #2323 - this PR lays the groundwork with the exception of what Siegfried mentioned in this issue (we have a prototype, it is rather simple). The reason for not moving to multi-tfm is that (a) our unit tests target netcore31, (b) the wapproj doesn't like multi-tfm. Not because this issue is hard.

[siegfriedpammer]

Once we have moved to .NET 5 or 6, we could adapt our deployment strategy to produce a framework-dependent binary release, which is then zipped (like today).