Fix SiteExtension loading on App Service (dotnet#44987)

Forward port of dotnet#44887

Author: BrennanConroy

src/SiteExtensions/Microsoft.Web.Xdt.Extensions/src/Microsoft.Web.Xdt.Extensions.csproj

@@ -11,7 +11,8 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <Reference Include="Microsoft.Web.Xdt" />
+    <!-- We need to reference the same version (or lower maybe) as App Service otherwise the custom transform cannot be loaded -->
+    <PackageReference Include="Microsoft.Web.Xdt" Version="1.4.0" AllowExplicitReference="true" />
   </ItemGroup>
 
 </Project>