Install .NET 6 SDK to re-enable XAML Styler

[Arlodotexe]

Fixes

This PR fixes the error observed at this CI run by installing the .NET 6 SDK explicitly.

Log for failed CI step:

Run powershell -version 5.1 -command "./ApplyXamlStyling.ps1 -Passive" -ErrorAction Stop
Use 'Help .\ApplyXamlStyling.ps1' for more info or '-Main' to run against all files.

Restoring dotnet tools...
Tool 'uno.check' (version '1.27.4') was restored. Available commands: uno-check
Tool 'xamlstyler.console' (version '3.2206.4') was restored. Available commands: xstyler
Tool 'microsoft.visualstudio.slngen.tool' (version '12.0.13') was restored. Available commands: slngen

Restore was successful.
Checking all files (passively)
You must install or update .NET to run this application.

App: C:\Users\runneradmin\.nuget\packages\xamlstyler.console\3.2206.4\tools\net6.0\any\xstyler.dll
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '6.0.0' (x64)
.NET location: C:\Program Files\dotnet\

The following frameworks were found:
  8.0.6 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  8.0.19 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  8.0.20 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  9.0.6 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  9.0.8 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  9.0.9 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

Learn more:
https://aka.ms/dotnet/app-launch-failed

To install missing framework, download:
https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=6.0.0&arch=x64&rid=win-x64&os=win10
Error: Process completed with exit code 1.

Other information

See also actions/runner-images#12241

[michael-hawker]

We should update XAML Styler version instead