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>

build/TargetFrameworks.props

@@ -0,0 +1,15 @@
+<Project>
+  <PropertyGroup>
+    <AvsCurrentTargetFramework>net8.0</AvsCurrentTargetFramework>
+    <AvsCurrentWindowsTargetFramework>$(AvsCurrentTargetFramework)-windows</AvsCurrentWindowsTargetFramework>
+    <AvsCurrentAndroidTargetFramework>$(AvsCurrentTargetFramework)-android34</AvsCurrentAndroidTargetFramework>
+    <AvsCurrentIOSTargetFramework>$(AvsCurrentTargetFramework)-ios17.0</AvsCurrentIOSTargetFramework>
+    <AvsCurrentTvOSTargetFramework>$(AvsCurrentTargetFramework)-tvos17</AvsCurrentTvOSTargetFramework>
+    <AvsCurrentBrowserTargetFramework>$(AvsCurrentTargetFramework)-browser</AvsCurrentBrowserTargetFramework>
+    <AvsCurrentTizenTargetFramework>$(AvsCurrentTargetFramework)-tizen</AvsCurrentTizenTargetFramework>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(AvsSkipBuildingLegacyTargetFrameworks)' != 'True'">
+    <AvsLegacyTargetFrameworks>net6.0</AvsLegacyTargetFrameworks>
+    <AvsLegacyWindowsTargetFrameworks>net6.0-windows</AvsLegacyWindowsTargetFrameworks>
+  </PropertyGroup>
+</Project>