.NET 6 upgrade (OSOE-60)

[Piedone]

Let's upgrade everything to .NET 6. Official docs: https://docs.microsoft.com/en-us/aspnet/core/migration/31-to-60

• The best is to wait for OC 1.3 with this since that's the version that'll drop .NET Core 3.1 support.
• NPM Targets and Gulp Extensions, due to being used in Orchard 1 and other .NET Framework projects, should target .NET Standard still.
• Search the whole code base not just for "netcoreapp3.1" but also "net5.0".

Related:

• .NET 6 upgrade (OSOE-60) .NET-Analyzers#29 is needed for this.
• Use WebApplicationFactory directly instead of running the tested app with the dotnet CLI (OSOE-46) UI-Testing-Toolbox#101 needs this.
• Remove direct System.Diagnostics.DiagnosticSource package reference Orchard-Azure-Application-Insights#27