Add .NET 8 target (Avalonia 11.1) (#14535)

* Add .NET 8 target to packable projects

* Update test projects

* Update sample projects to target .NET 8

* Update main Avalonia package to target .NET 8 as well

* Remove MSBuildEnableWorkloadResolver hack and some minor change

* Use net8.0 in nuke build as well

* Pin 8.0.0 SDK

* Adjust API validation

* Pin a valid version

* Remove net461 target from Avalonia package

* Remove unused net6.0 target from Avalonia.Designer.HostApp

* Adjust API diff

* Update ControlCatalog.Android.csproj

* Remove MSBuildEnableWorkloadResolver

* Fix Browser issues on .NET 8

* Fix .NET 8 error

* Fix merge conflicts

* Replace explicit TFMs in .csproj files with a centralized registry

* Fix merge conflict

* Fix AvsLegacyWindowsTargetFrameworks and add hacks for the tests

* Update XCode project CommandLineArgument

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Co-authored-by: Nikita Tsukanov <keks9n@gmail.com>

Directory.Build.props

@@ -1,11 +1,11 @@
 <Project>
   <Import Project="$(MSBuildThisFileDirectory)/build/AvaloniaPublicKey.props"/>
+  <Import Project="$(MSBuildThisFileDirectory)/build/TargetFrameworks.props"/>
   <PropertyGroup>
       <PackageOutputPath Condition="'$(PackageOutputPath)' == ''">$(MSBuildThisFileDirectory)build-intermediate/nuget</PackageOutputPath>
       <AvaloniaPreviewerNetCoreToolPath>$(MSBuildThisFileDirectory)\src\tools\Avalonia.Designer.HostApp\bin\$(Configuration)\netstandard2.0\Avalonia.Designer.HostApp.dll</AvaloniaPreviewerNetCoreToolPath>
       <!-- https://github.com/dotnet/msbuild/issues/2661 -->
       <AddSyntheticProjectReferencesForSolutionDependencies>false</AddSyntheticProjectReferencesForSolutionDependencies>
-      <MSBuildEnableWorkloadResolver>false</MSBuildEnableWorkloadResolver>
       <RunApiCompat>False</RunApiCompat>
       <LangVersion>11</LangVersion>
   </PropertyGroup>