Add Change record to @deprecated for config.storage.staging in core.services.yml

I confirm the change is correct.

Additionnally, the function getStaging seems had been removed in the class FileStorageFactory so it's not necessary to document it.

As described in #2887156: Add change record link @deprecated for the CMI staging, there are three changes included in https://www.drupal.org/node/2574957. One (the constant) has already been documented in #2873749: Add change record links to @deprecated items in bootstrap.inc, the second (the method) was just directly renamed and so does not need documentation, and the third (the service) is covered in this patch.

My one question was whether we should use @see in a YAML file, but we already do this in several other places in core, so that makes sense too.

Thanks @Calystod and @kwhite! Committed and pushed to 8.4.x. I also backported this to 8.3.x since it is a documentation improvement (reference: https://www.drupal.org/core/d8-allowed-changes#patch).

Removing parent issue per conversation with @xjm at Drupalcon Nashville Mentored Sprint prep. Her recommendation to do so was based on a few points that made sense to all of us in the discussion, namely:
- so many child issues makes this parent unwieldy
- search filters will allow people needing to refind closed children