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

---------

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

azure-pipelines.yml

@@ -36,7 +36,7 @@ jobs:
       version: 6.0.x
 
   - task: UseDotNet@2
-    displayName: 'Use .NET 7.0 SDK'
+    displayName: 'Use .NET 8.0 SDK'
     inputs:
       packageType: sdk
       useGlobalJson: true
@@ -82,7 +82,7 @@ jobs:
       version: 6.0.x
 
   - task: UseDotNet@2
-    displayName: 'Use .NET 7.0 SDK'
+    displayName: 'Use .NET 8.0 SDK'
     inputs:
       packageType: sdk
       useGlobalJson: true
@@ -162,7 +162,7 @@ jobs:
       version: 6.0.x
 
   - task: UseDotNet@2
-    displayName: 'Use .NET 7.0 SDK'
+    displayName: 'Use .NET 8.0 SDK'
     inputs:
       packageType: sdk
       useGlobalJson: true