Update TFM from .netcoreapp5.0 -> .net5.0

[AdamBraden]

Need to move to the updated TFM for .net5.0

[Scottj1s]

@AdamBraden, is this safe now? For what version(s) of .NET 5 SDK and Visual Studio?

[azchohfi]

It should be safe:
https://devblogs.microsoft.com/aspnet/asp-net-core-updates-in-net-5-preview-3/

Change the TFM in your *.csproj file from netcoreapp5.0 to net5.0

[Scottj1s]

Turns out it's not safe, quite yet.

Latest public nuget.exe, v.5.5.1, results in 5 build breaks like this:
error : Project '..\WinUIProjection\WinUIProjection.csproj' targets 'net5.0'. It cannot be referenced by a project that targets '.NETCoreApp,Version=v5.0'. [C:\git\cswinrt2\WinUI\WinUITest\WinUITest.csproj]

The latest pre-release nuget.exe, v5.6.0-preview2, reduces that number of breaks to 2.

Per .NET team, using 'msbuild /t:restore' from VS 16.6 p2 or later is required. But that doesn't work for packages.config-based vcxproj. The workaround is to split out restores based on project type: msbuild for csproj, and nuget for vcxproj.

In VS, the nuget console reports these versions:
VS 16.5.4: 5.5.0.6473
VS 16.6 p4: 5.6.0.6558
So it may be necessary for a nuget.exe v5.6.0.6558 or better to work for command-line builds.

Until VS 16.6 is deployed to hosted build agents, or the nuget client supports net5.0 TFM, in a holding pattern.

Update: turns out nuget v5.6 is necessary but not sufficient. The build breaks reference VS 16.6 targets files.

[Scottj1s]

VS 16.6 rollout on Azure Devops has been very nearly complete (~99%) for several days. I'll go ahead and complete the PR now so dependents can start to move forward.