Fix dotnet-6 tag to actually target .net 6

[robertbjarum]

Please create a new tag dotnet-6 that actually targets .NET 6 and not .NET 7.

I downloaded the asset dotnet-6.zip from GitHub https://github.com/ardalis/CleanArchitecture/archive/refs/tags/dotnet-6.zip

Noticed that all the projects were targeting .NET 7.

I want to stick with .NET 6 a bit longer, so I modified the project files to target .NET 6 as expected.

However, then I get build error due to incompatible Microsoft.NET libraries.

Modified Directory.Packages.props from 7.0.0 to

<PackageVersion Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="6.0.0" />
<PackageVersion Include="Microsoft.VisualStudio.Web.CodeGeneration.Design" Version="6.0.0" />

Then it builds.