Update to MAUI .NET 9 Preview 6 manifests

[jonathanpeppers]

Context: https://github.com/dotnet/maui/releases/tag/9.0.0-preview.6.24327.7
Context: https://github.com/dotnet/android/releases/tag/34.99.0-preview.6.340
Context: https://github.com/xamarin/xamarin-macios/releases/tag/dotnet-9.0.1xx-preview6-9714

These released yesterday with .NET 9 Preview 6.

[jonathanpeppers]

I have not done this since dotnet/installer moved into dotnet/sdk.

I ran .\build.cmd -pack -publish, but I don't see my changes:

> .\artifacts\bin\redist\Debug\dotnet\dotnet.exe workload list

Installed Workload Id      Manifest Version                             Installation Source
-------------------------------------------------------------------------------------------
macos                      14.2.9088-net9-p1/9.0.100-preview.1          VS 17.10.35013.160
maccatalyst                17.2.9088-net9-p1/9.0.100-preview.1          VS 17.10.35013.160
ios                        17.2.9088-net9-p1/9.0.100-preview.1          VS 17.10.35013.160
android                    34.99.0-preview.1.151/9.0.100-preview.1      VS 17.10.35013.160
maui-windows               9.0.0-preview.1.9973/9.0.100-preview.1       VS 17.10.35013.160

These still show preview.1:

> .\artifacts\bin\redist\Debug\dotnet\dotnet.exe workload install android --skip-manifest-update

Installing pack Microsoft.Android.Sdk.Windows version 34.99.0-preview.1.151...

[jonathanpeppers]

After looking at a .binlog, I should have been testing with .\artifacts\bin\redist-installer\Debug\dotnet\dotnet.exe for this.