Ignore migration fields in ContainerRepository

What does this MR do and why?

Issue: Item 3️⃣ in the issue [container registry migration] Remove backend r... (#409873)

In this MR, we set the migration fields on ContainerRepository as ignored columns. This is the first step in removing them as per the guidelines.

To ignore the field, migration_state, we are deprecating it from GraphQL. We are returning an empty string ('') as suggested in #409873 (comment 1885668305).

The migration is now complete and these fields are no longer used.

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

Head over to the GraphQL explorer: http://gdk.test:3000/-/graphql-explorer

Try the query below on master and on this branch.

query {
  containerRepository(id: "gid://gitlab/ContainerRepository/13") {
    migrationState
  }
}

On master, it would return a migration state: (i.e. default, migration_done).

{
  "data": {
    "containerRepository": {
      "migrationState": "default"
    }
  }
}

On this branch, it would return an empty string:

{
  "data": {
    "containerRepository": {
      "migrationState": ""
    }
  }
}

Related to #409873