Show storage pre-enforcement banner only to impacted namespaces

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/367531

This show's the banner only to affected namespaces, i.e. the namespace has 5+ GB of storage

Screenshots or screen recordings

There are no UI changes, so putting just the banner for reference

(Obs: some images show usage below 5GB because I didn't mock the data in my GDK in all scenarios)

User accessing their own namespace
User as a project maintainer on other user's namespace
Subgroup
Project
Root Group

How to set up and validate locally

1. Create a new Group, take note of the Group ID (e.g. 123)
2. Make sure the banner does NOT show on this group's page
3. Add namespace storage that reaches the limit

    n = Group.find(166)
    n.root_storage_statistics&.destroy!
    s = Namespace::RootStorageStatistics.new(
      namespace: n,
      build_artifacts_size: 0,
      wiki_size: 0,
      repository_size: 0,
      packages_size: 0,
      lfs_objects_size: 5.gigabytes
    )
    s.storage_size = s.repository_size + s.lfs_objects_size + s.wiki_size + s.build_artifacts_size + s.packages_size
    s.save!

4. Make sure the banner appears

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.