Use dashboard limit on storage alerts

What does this MR do and why?

This implements https://gitlab.com/gitlab-org/gitlab/-/issues/431395

This MR is dependent on !138331 (merged)

Screenshots or screen recordings

Before	After	After with purchased storage

How to set up and validate locally

Use these commands in rails console:

1. Gitlab::CurrentSettings.update!(automatic_purchased_storage_allocation: true)
2. Gitlab::CurrentSettings.update!(enforce_namespace_storage_limit: true)
3. Plan.free.actual_limits.update!(notification_limit: 5*1024)
4. Plan.free.actual_limits.update!(enforcement_limit: 10*1024)
5. Plan.free.actual_limits.update!(storage_size_limit: 5*1024)
6. Project.find(29).statistics.update(repository_size: 12.gigabytes)
   1. Sidekiq.redis(&:flushdb)
   2. Namespaces::ScheduleAggregationWorker.perform_sync(89)
   3. Namespaces::RootStatisticsWorker.perform_sync(89)
7. Feature.enable(:namespace_storage_limit_show_preenforcement_banner, Group.find(89))
8. Feature.enable(:namespace_storage_limit, Group.find(89))

Check the number in the banner is storage_size_limit and not enforcement_limit.

MR acceptance checklist

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