Unified Backups: Introduce `backup_information.json` metadata

What does this MR do and why?

This introduces a new metadata format for the Unified Backups: backup_information.json.

The change from old file in YAML to new file being in JSON is to allow defining a JSON Schema to validate the file in a later stage.

This also makes the two backups incompatible, which removes the need to implement a different validation mechanism on the old one.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

When a Backup is performed, it should include on the root folder a file: backup_information.json with the following keys:

• metadata_version
• backup_id
• created_at
• gitlab_version

{
  "metadata_version": 2,
  "backup_id": "1712944480_2024_04_12_16.11.0-pre",
  "created_at": "2024-04-12T17:54:40Z",
  "gitlab_version": "16.11.0-pre"
}

Related to #427337 (closed)