Add change record link @deprecated for the CMI staging

I added a link for the record change about the deprecated CMI staging.

This change concern the issue #2487588: Move CMI import/export directory "staging" to "sync", as it is confused with staging environments and the record change page is https://www.drupal.org/node/2574957

Apparently, the link for the constant had been already add with the issue #2873749: Add change record links to @deprecated items in bootstrap.inc

I add the link for the deprecated service but for the function getStaging, it seems had been removed in the class FileStorageFactory.

I add my patch for the file core.service.yml.