Requires .NET 6 which is out of support

I only have .NET 8 (LTS) and 9 installed on my machine. When I try to run the tool, I get this error message:

```
You must install or update .NET to run this application.
```

I cannot use the tool, unless I specify `DOTNET_ROLL_FORWARD` environment variable:

```
DOTNET_ROLL_FORWARD=LatestMajor dotnet packagesourcemapper generate nuget.config
```

Please update the tool to .NET 8.