Add explicit reference to System.Net.Sockets

src/Abc.Zebus/Abc.Zebus.csproj

@@ -18,6 +18,9 @@
     <PackageReference Include="Newtonsoft.Json" Version="11.0.2" />
     <PackageReference Include="protobuf-net" Version="2.3.13" />
     <PackageReference Include="structuremap" Version="4.6.1" />
+
+    <!-- Avoid "NU1605: Detected package downgrade" error -->
+    <PackageReference Include="System.Net.Sockets" Version="4.3.0" />
   </ItemGroup>
 
   <ItemGroup>
@@ -40,4 +43,4 @@
     <EmbeddedResource Include="Transport\libzmq-x86.dll" />
   </ItemGroup>
 
-</Project>
+</Project>