AvaloniaLanguageServer.csproj:Update to .NET 9.0 to fix errors

[4Darmygeometry]

If use .NET 9.0 to run Avalonia for vscode, will be errors:

App: /home/gamma/.vscode-oss/extensions/avaloniateam.vscode-avalonia-0.0.26-universal/solutionParserTool/SolutionParser.dll
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '8.0.0' (x64)
.NET location: /home/gamma/.dotnet/

The following frameworks were found:
  9.0.3 at [/home/gamma/.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=8.0.0&arch=x64&rid=linux-x64&os=aosc.12.1.0```

[frostblue]

Any progress on merging this PR? I'm having the same issue. It's technically not this extension's fault as it works fine with .NET 8, but I'm using OpenSUSE Tumbleweed and the .NET 8 package from the Microsoft repos is currently broken which means I cannot use the language server with an official .NET install.

[lextm]

We cherry-picked your change in this fork and shipped as release 11.0.35.