.Net 6.0 is outdated, upgrade to .net 8

In my experiment https://github.com/davidkhala/action-tests/actions/runs/12671257519/job/35312624018

I see errors like

```
C:\Windows\system32\cmd.exe /D /S /C "D:\a\_msdo\versions\microsoft.security.devops.cli\0.212.3\tools\guardian.cmd init --force"
You must install or update .NET to run this application.

App: D:\a\_msdo\versions\microsoft.security.devops.cli\0.2[12](https://github.com/davidkhala/action-tests/actions/runs/12671257519/job/35312624018#step:4:13).3\tools\Microsoft.Guardian.Cli.exe
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '8.0.0' (x64)
.NET location: C:\Users\runneradmin\AppData\Local\Microsoft\dotnet

The following frameworks were found:
6.0.36 at [C:\Users\runneradmin\AppData\Local\Microsoft\dotnet\shared\Microsoft.NETCore.App]
```

There are several documents and samples we should change and align with latest workable status