Support net5.0

[ViktorHofer]

This item tracks the required work for supporting net5.0. This issue doesn't track supporting the OS part of the TargetFramework, i.e. net5.0-ios.

Critical path: NuGet --> SDK --> Template

Work Items:

• NuGet - Understanding net5.0 as ".NETCoreApp" TargetFrameworkIdentifier (TFI)
  Tracking issue: Initial Net5.0 TFM support NuGet/Home#9089
  Changeset: NuGet/NuGet.Client@162e235, Assignee: @rrelyea
• SDK - Understanding net5.0 as ".NETCoreApp" TFI
  Changeset: 0f5fc4b
  Assignee: @dsplaisted
• Templating - Change the default TFM to net5.0
  Assignee: @ViktorHofer
  • test templates: Update .NET 5 templates with net5.0 tfm test-templates#83
  • nunit3 template: replace netcoreapp5.0 to net5.0 due to changes in .NET Core SDK nunit/dotnet-new-nunit#39
  • lib & console: Update .NET 5 templates with net5.0 tfm templating#2310
  • web templates: Default web templates to net5.0 aspnetcore#19860
  • windowsdesktop templates: Change tfm to net5.0 windowsdesktop#800, Update templates to net5.0 winforms#3344, Update .NET 5 templates with net5.0 tfm wpf#3042
• React to TFM change in installers (runtime, windowsdesktop, aspnetcore), @ViktorHofer
• Scouting the net5.0 change, @ViktorHofer @dsplaisted @rrelyea