[WIP] Use NET=Runtime on tasks and delete .NET Framework task implementations

[ViktorHofer]

By using NET="Runtime" on UsingTask declarations, msbuild runs the task always on .NETCoreApp, even on desktop msbuild.

Delete two unused projects:

• IssueManager (not used in dnceng anymore)
• Microsoft.DotNet.Tar (was only necessary for desktop SignTool)

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Benefits:

• Less code to maintain
• Less Audit/CG alerts due to the removal of .NET Framework toolset version dependencies

[ViktorHofer]

Two blockers so far:

• Enable .NET TaskHost usage for Project types that do not import the base .NET SDK-delivered MSBuild SDK msbuild#12895 (for pkgproj / helix / other project types that don't import Microsoft.NET.SDK), cc @baronfel
• SignTool and SignCheckTool expect that the task / lib runs on .NET Framework. I remember @mmitche mentioning that he was looking into this. Naively speaking, the .NETCoreApp task/lib running on Windows should have the required APIs / feature sets available?

Slightly related: There a few remaining tools(!) / libraries that still target .NET Framework that I'm not sure about: Helix.Client, Helix.JobSender, StrongName, SignCheck and SignCheckLibrary. I wonder if all those need to stay on .NET Framework?

[mmitche]

Love this.

[mmitche]

SignTool running on framework might have to do with packing/unpacking VSIX's. At least that was the case at some point. For signcheck...IIRC some functionality used to not be supported on core. Maybe some authenticode or VSIX checking? Either way, I think that was resolved.