Fixed System.Text.Json version mismatch warning

Author: xperiandri

tests/FSharp.Data.GraphQL.IntegrationTests/FSharp.Data.GraphQL.IntegrationTests.fsproj

@@ -10,6 +10,7 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.Http" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="System.Text.Json" />
     <PackageReference Include="xunit" />
     <PackageReference Include="xunit.runner.visualstudio" />
   </ItemGroup>