chore(deps): updated Directory.Build.props file

changes:
- removed the support for `net6.0`
- added the support for `net9.0`

Directory.Build.props

@@ -29,7 +29,7 @@
     <NeutralLanguage>en-US</NeutralLanguage>
     <Nullable>enable</Nullable>
     <SignAssembly>true</SignAssembly>
-    <TargetFrameworks>netstandard2.0;net462;net6.0;net8.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net462;net481;net8.0;net9.0</TargetFrameworks>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)'=='Release'">
@@ -63,7 +63,7 @@
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
     <GenerateDocumentationFile>false</GenerateDocumentationFile>
-    <TargetFrameworks>net462;net6.0;net8.0</TargetFrameworks>
+    <TargetFrameworks>net462;net481;net8.0;net9.0</TargetFrameworks>
   </PropertyGroup>
 
   <ItemGroup Condition="!$(MSBuildProjectName.EndsWith('Tests'))">