Fix one more version conflict on the docs transport package (#6675)

Author: joperezr

src/Packages/Microsoft.Internal.Extensions.DotNetApiDocs.Transport/Microsoft.Internal.Extensions.DotNetApiDocs.Transport.proj

@@ -32,6 +32,7 @@
     and some others depending on the 9.0 version. -->
     <PackageReference Include="System.Text.Encodings.Web" />
     <PackageReference Include="System.Diagnostics.DiagnosticSource" />
+    <PackageReference Include="System.Text.Json" />
   </ItemGroup>
 
   <!-- Include the reference assembly and put the documentation file next to it. -->