Updated to System.Json.Text 5.0.0

This provides access to .NET 5 attributes, such as `[JsonInclude]` and `[JsonIgore()]` which we were previously relying upon .NET 5 for—but without the need to multi-target .NET 5.

Author: JeremyCaney

OnTopic.Data.Transfer/OnTopic.Data.Transfer.csproj

@@ -36,7 +36,7 @@
     </PackageReference>
     <PackageReference Include="OnTopic" Version="5.0.0-beta.1" />
     <PackageReference Include="System.ObjectModel" Version="4.3.0" />
-    <PackageReference Include="System.Text.Json" Version="4.6.0" />
+    <PackageReference Include="System.Text.Json" Version="5.0.0" />
   </ItemGroup>
 
 </Project>