chore: centralize avalonia project settings.

src/Avalonia/Directory.Build.props

@@ -1,6 +1,8 @@
 <Project>
   <PropertyGroup>
     <Nullable>enable</Nullable>
+    <LangVersion>latest</LangVersion>
+    <TargetFramework>net9.0</TargetFramework>
     <AvaloniaVersion>11.3.6</AvaloniaVersion>
   </PropertyGroup>
 </Project>

src/Avalonia/HandyControlDemo_Avalonia/HandyControlDemo_Avalonia.csproj

@@ -3,9 +3,6 @@
     <OutputType>WinExe</OutputType>
     <!--If you are willing to use Windows/MacOS native APIs you will need to create 3 projects.
     One for Windows with net7.0-windows TFM, one for MacOS with net7.0-macos and one with net7.0 TFM for Linux.-->
-    <TargetFramework>net7.0</TargetFramework>
-    <Nullable>enable</Nullable>
-    <LangVersion>latest</LangVersion>
     <BuiltInComInteropSupport>true</BuiltInComInteropSupport>
     <ApplicationManifest>app.manifest</ApplicationManifest>
     <AssemblyName>HandyControlDemo</AssemblyName>

src/Avalonia/HandyControl_Avalonia/HandyControl_Avalonia.csproj

@@ -1,8 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net7.0</TargetFramework>
-    <Nullable>enable</Nullable>
-    <LangVersion>latest</LangVersion>
     <AssemblyName>HandyControl</AssemblyName>
     <RootNamespace>HandyControl</RootNamespace>
   </PropertyGroup>