Modify storage limit to use admin controlled value

What does this MR do and why?

Updates Namespaces::Storage::Enforcement and Namespaces::Storage::RootSize classes to use the admin controlled limit values to determine:

1. if a storage limit should apply
2. what that limit should be

Previously, storage limits were going to be controlled via a mix of feature flags and plan_limit.storage_size_limit, but this is no longer the case.

Now, Admins will be able to set an enforcement limit (plan_limit.enforcement_limit) and a dashboard limit (plan_limit.storage_size_limit) (see: https://gitlab.com/groups/gitlab-org/-/epics/9173 for high level overview), and we need to use these to determine the answers to the above.

In short, the rules are:

• storage limit application settings and feature flags must be enabled
• namespace is on the free plan
• the namespace has not been excluded from storage limits
• either of:
  • an storage_size_size_limit ("dashboard limit") is set, and the dashboard limit is enabled (dashboard_limit_enabled_at) and the namespace was created after it was enabled (see https://gitlab.com/gitlab-org/gitlab/-/issues/385728#considerations for more info/examples)
  • an enforcement_limit value is set to > 0

Otherwise, we fallback to having a limit of 0, which indicates unlimited

How to set up and validate locally

1. Checkout master branch to test behaviour before this MR
2. Toggle on the various options

   ::Gitlab::CurrentSettings.update automatic_purchased_storage_allocation: true
   
   ::Gitlab::CurrentSettings.update enforce_namespace_storage_limit: true
   
   Feature.enable :namespace_storage_limit

3. Find a free group on your local instance

   group = Group.find(#id of your free-plan group)

4. Check enforcement logic against the group:

   Namespaces::Storage::Enforcement.enforce_limit?(group)
   
   => false

5. Checkout this branch (vij-admin-storage-limit) and retry the above to confirm the same behaviour without any plan limit changes:

   reload!
   
   group = Group.find(#id)
   
   Namespaces::Storage::Enforcement.enforce_limit?(group)
   
   => false

6. Now, set an enforcement limit and try again:

   group.actual_limits.update(enforcement_limit: 1) # sets the enforcement limit to 1kb
   
   Namespaces::Storage::Enforcement.enforce_limit?(group)
   
   => true

7. Check the dashboard limit rules:

   group.actual_limits.update(enforcement_limit: 0, storage_size_limit: 1, dashboard_limit_enabled_at: group.created_at - 1.day) 
   
   Namespaces::Storage::Enforcement.enforce_limit?(group)
   
   => true

8. Temporarily disable caching so we can test the root size limit by modifying this method:

       def limit
        #@limit ||= Rails.cache.fetch(limit_cache_key, expires_in: EXPIRATION_TIME) do
          storage_enforcement.enforceable_storage_limit(root_namespace).megabytes +
            root_namespace.additional_purchased_storage_size.megabytes
        #end
      end
      
      # don't forget to `reload!` the rails console to make sure this takes effect

9. Lastly, we can check the RootSize class limit works as intended:

   group.actual_limits.update(enforcement_limit: 0, storage_size_limit: 0) 
   
   Namespaces::Storage::RootSize.new(group).limit
   
   => 0
   
   group.actual_limits.update(enforcement_limit: 10) 
   Namespaces::Storage::RootSize.new(group).limit
   
   => 10485760 # returned in KiB
   
   group.actual_limits.update(enforcement_limit: 0, storage_size_limit: 10, dashboard_limit_enabled_at: group.created_at - 1.day) 
   Namespaces::Storage::RootSize.new(group).limit
   
   => 10485760 # returned in KiB
   
   group.actual_limits.update(enforcement_limit: 20, storage_size_limit: 10, dashboard_limit_enabled_at: group.created_at - 1.day) 
   Namespaces::Storage::RootSize.new(group).limit
   
   => 10485760 # still returns dashboard limit

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.