Bump sample to .NET 9 (using new LoadIntoBufferAsync overload)

Author: MihaZupan

snippets/csharp/System.Net.Http/HttpCompletionOption/HttpCompletionOptionSnippets.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFramework>net9.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
   </PropertyGroup>