Fixes #10280

Context

.NET 9 is in preview 6.
Consuming the latest Arcade + retargeting the the msbuild core version to .net 9.

Changes Made

• Upgrade the Target framework from .net8 to .net9
• darc update-dependencies --channel ".NET Eng - Latest" --name "Microsoft.DotNet.Arcade.Sdk"
• darc update-dependencies --channel ".NET Eng - Latest" --name "Microsoft.SourceBuild.Intermediate.arcade"
• Version of xunit provided by arcade.sdk reverted, not to introduce the changes in one PR.
• ApiCompat warnings supressed by dotnet pack /p:GenerateCompatibilitySuppressionFile=true
• Addressing affected test behavior by .NET 9 Preview 6 breaking change: https://learn.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/9.0/empty-env-variable. To delete the env var we need to pass the null value
• Addressed [Kitten] Remove APICompat workaround once 8.0.2 or 9.xxx is consumed #9720

Testing

• Existing tests should pass
• Exp insertion: https://devdiv.visualstudio.com/DevDiv/_git/VS/pullrequest/569783

Next steps

• Update subscriptions to Microsoft.DotNet.Arcade.Sdk from .NET ENG 8 To ".NET Eng - Latest"
• Unfreeze the xunit version #10557
• Change xliff-tasks dependency in Version.Details.xml when consuming Arcade 9 #9366