[WPF] Fix lang version to default (#3199)

Xamarin.Forms.Platform.WPF/Xamarin.Forms.Platform.WPF.csproj

@@ -22,7 +22,7 @@
     <DefineConstants>TRACE;DEBUG</DefineConstants>
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
-    <LangVersion>7</LangVersion>
+    <LangVersion>default</LangVersion>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
     <DebugType>pdbonly</DebugType>
@@ -31,7 +31,7 @@
     <DefineConstants>TRACE</DefineConstants>
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
-    <LangVersion>6</LangVersion>
+    <LangVersion>default</LangVersion>
   </PropertyGroup>
   <ItemGroup>
     <Reference Include="OpenTK, Version=2.0.0.0, Culture=neutral, PublicKeyToken=bad199fe84eb3df4, processorArchitecture=MSIL">