Update KubernetesClient DiagnosticSource dependency (#1561)

System.Diagnostics.DiagnosticSource has been included into the framework reference, no need to reference for net8

Author: WeihanLi

src/KubernetesClient/KubernetesClient.csproj

@@ -7,7 +7,7 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="System.Diagnostics.DiagnosticSource" Version="7.0.0" />
+        <PackageReference Condition="'$(TargetFramework)'=='net6.0'" Include="System.Diagnostics.DiagnosticSource" Version="7.0.0" />
         <PackageReference Include="System.IdentityModel.Tokens.Jwt" Version="7.1.2" />
         <PackageReference Include="IdentityModel.OidcClient" Version="5.2.1" />
         <PackageReference Include="Fractions" Version="7.3.0" />