Resolve NU1510 as System.Net.ServerSentEvents is inbox now (#61467)

Unblocks dotnet/sdk#48395

Author: ViktorHofer

src/SignalR/clients/csharp/Http.Connections.Client/src/Microsoft.AspNetCore.Http.Connections.Client.csproj

@@ -25,7 +25,7 @@
     <Reference Include="Microsoft.AspNetCore.Http.Connections.Common" />
     <Reference Include="Microsoft.Extensions.Logging.Abstractions" />
     <Reference Include="Microsoft.Extensions.Options" />
-    <Reference Include="System.Net.ServerSentEvents" />
+    <Reference Include="System.Net.ServerSentEvents" Condition="'$(TargetFramework)' != '$(DefaultNetCoreTargetFramework)'" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == '$(DefaultNetFxTargetFramework)' OR '$(MSBuildRestoreSessionId)' == ''">