Dotnet New Nunit Versions Save

• Add .NET 6.0 and make the default
• update NUnit3TestAdapter to v4.2.1 on .NET Core 2.1+
• update NUnit to v3.13.3
• update Microsoft.NET.Test.Sdk to v7.1.0 on .NET 6.0+
• Removed EOL .NET Core versions

Replaced netcoreapp5.0 to net5.0 due to changes in 5.0.100-preview.2.20176.6 version of .NET Core SDK (https://github.com/nunit/dotnet-new-nunit/issues/38)

• add new --framework supported parameters:
  • netcoreapp5.0

• update NUnit3TestAdapter to v3.16.1 (supported for .NET Core 2.1+)
• update Microsoft.NET.Test.Sdk dependency to v16.5.0 (supported for .NET Core 2.0+)

• update Microsoft.NET.Test.Sdk dependency to v16.4.0
• fix condition to include latest version of Microsoft.NET.Test.Sdk for netcoreapp2 and higher

• update Microsoft.NET.Test.Sdk dependency to v16.4.0
• fix condition to include latest version of Microsoft.NET.Test.Sdk for netcoreapp2 and higher

• add new --framework supported parameters:
  • .NET Core 3.1 version netcoreapp3.1
• update all project templates to default to .NET Core 3.1 (netcoreapp3.1). Other frameworks can still be selected with the --framework command line option.

• update NUnit3TestAdapter to v3.15.1
• update Microsoft.NET.Test.Sdk to v16.3.0

Note: Microsoft.NET.Test.Sdk v16.3.0 has dropped support for .NET Core 1., therefore, when you targeting .NET Core 1., you'll stay with Microsoft.NET.Test.Sdk to v16.2.0.

• update nunit dependency to v3.12.0
• update Microsoft.NET.Test.Sdk dependency to v16.2.0
• F# unit test templates are made module based instead of class based (thanks to @zanaptak, #27)

• fix default namespace based on project name
• update NUnit3TestAdapter dependency to v3.13.0
• update Microsoft.NET.Test.Sdk dependency to v16.0.1