Add all ApplicationSettings attributes to the API

What does this MR do?

This makes the list of visible attributes for ApplictionSettings shared across the UI & the API

Are there points in the code the reviewer needs to double check?

I deprecated an attribute in the API that should already have been removed repository_storage in favor of repository_storages

Why was this MR needed?

Because not all new applicationsettings were being exposed through the api

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing

What are the relevant issue numbers?

First part of https://gitlab.com/gitlab-org/gitlab-ce/issues/34163, needs an EE counterpart.