Add placeholder weights for newly added repository storages

What does this MR do and why?

We had a problem where, when we updated gitlab.yml with a new repository storage the new storage wasn't showing up in api/v4/application/settings. This MR fixes it by using gitlab.yml as SSOT to list the storage.

How to set up and validate locally

1. Update gitlab.yml with a new repository storage
2. Restart GitLab so it reads the new configuration
3. Observe that the repository is listed in the web ui
4. Observe that the repository storage is listed when you retrive storages using the settings api

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #426091