Bump LangVersion to C# 12 (#15399)

Directory.Build.props

@@ -7,6 +7,6 @@
       <!-- https://github.com/dotnet/msbuild/issues/2661 -->
       <AddSyntheticProjectReferencesForSolutionDependencies>false</AddSyntheticProjectReferencesForSolutionDependencies>
       <RunApiCompat>False</RunApiCompat>
-      <LangVersion>11</LangVersion>
+      <LangVersion>12</LangVersion>
   </PropertyGroup>
 </Project>

samples/Directory.Build.props

@@ -4,7 +4,7 @@
     <IsPackable>false</IsPackable>
     <AvaloniaPreviewerNetCoreToolPath>$(MSBuildThisFileDirectory)..\src\tools\Avalonia.Designer.HostApp\bin\Debug\netstandard2.0\Avalonia.Designer.HostApp.dll</AvaloniaPreviewerNetCoreToolPath>
     <EnableNETAnalyzers>false</EnableNETAnalyzers>
-    <LangVersion>11</LangVersion>
+    <LangVersion>12</LangVersion>
     <NoWarn>$(NoWarn);CS8002</NoWarn> <!-- ignore signing warnings for samples -->
   </PropertyGroup>
 

samples/SafeAreaDemo/SafeAreaDemo.csproj

@@ -2,7 +2,6 @@
   <PropertyGroup>
     <TargetFramework>$(AvsCurrentTargetFramework)</TargetFramework>
     <Nullable>enable</Nullable>
-    <LangVersion>latest</LangVersion>
     <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>
   </PropertyGroup>