Fix C# sample build failures by upgrading to compatible .NET versions (#852)

* Initial plan

* Fix C# sample build failures by upgrading to compatible .NET versions

Co-authored-by: vicancy <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: vicancy <[email protected]>

Author: Copilot

samples/csharp/chatapp-net6/chatapp-net6.csproj

@@ -9,7 +9,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.Azure.WebPubSub.AspNetCore" Version="1.*" />
+    <PackageReference Include="Microsoft.Azure.WebPubSub.AspNetCore" Version="1.4.0" />
   </ItemGroup>
 
 </Project>

samples/csharp/chatapp-withstorage/chatwithstorage.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <RootNamespace>chatwithstorage</RootNamespace>

samples/csharp/clientWithCert/server/server.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <UserSecretsId>ClientWithCert.Server</UserSecretsId>