Build for dotnet 3.1 no longer possible

[wofsauge]

Brief Description

Since the commit db7949b was added, it is no longer possible to run the "build.sh" file with only dotnet 3.1 installed. This dotnet version is required for our project, so an upgrade to the now "enforced" dotnet v6.0 version is not possible right now.

I would propose that there could be a command line attribute to choose between dotnet 3.1 and 6.0.

Steps to reproduce

1. Install dotnet 3.1 ./dotnet-install.sh -c 3.1
2. checkout git repo git clone https://github.com/mono/CppSharp.git
3. run ./build.sh -configuration Release -platform x64
4. The build script will not finish and will write errors like shown in the Stack trace section below

The bug will not occur, when checking out the older commit c4a24b0.

OS: Linux (Ubuntu 20.04.5)

Target: MSVC/GCC/Clang

Stack trace or incompilable generated code

/home/myUser/.dotnet/sdk/3.1.426/Microsoft.Common.CurrentVersion.targets(1177,5): error MSB3971: The reference assemblies for ".NETFramework,Version=v6.0" were not found. You might be using an older .NET SDK to target .NET 5.0 or higher. Update Visual Studio and/or your .NET SDK. [/home/myUser/testcpp/CppSharp/tests/VTables/VTables.Tests.CSharp.csproj]

[tritao]

You can now pass an explicit version to build.

[wofsauge]

awesome! thank you very much for the quick fix :)