[Reseller] Update "change linked namespace" flow

## Problem

Currently, it's possible that if the customer tries to change the linked namespace to a namespace with more seats in use than seats in the subscription, they will be asked to pay for the additional seats.  

We cannot allow this transaction in Customers Portal, since customer should be transacting through a Reseller.

### How Support handles this today

When support encounters this situation, they:

1. Inform the customer that they have more users than purchased seats.
2. Propose the following options:
   1. Contact their purchase partner to purchase more seats.
   2. Deal with overages at the end of the subscription term (via True-Ups).
   3. Remove excess users.
3. Depending on what option was selected, wait for the daily sync job to run and re-calculate the Billable users count.
4. Link the subscription and confirm there are no overages recorded.

## Proposal

Update the `Change Linked Namespace` flow for a subscription that is purchased via a reseller.

- **If they have more users in their group than purchased seats**, display a message about the overage and let the user know that they will be charged for the overage at the end of their subscription term. Allow them to associate the subscription to the namespace regardless of overages.
- **If there is no overage** for a reseller customer, we will use the current flow (no changes needed).

| Current "Change linked namespace" flow (No overage) | Current "Change linked namespace" flow (With overage)  | Proposed reseller customer "Change linked namespace" flow (With overage) |
| ------ | ------ |  ------ |
| ![image](/uploads/2366421c283f5af7917a5db837077481/image.png) | ![image](/uploads/d8331298970fe91669fc7ec270981c80/image.png) | ![image](/uploads/16db327e08bafbfb05510c490bcff64b/image.png) |

### Design

- Mockup in design section https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5015/designs/Change_linked_namespace_for_reseller_customers.png
- [Figma file](https://www.figma.com/file/8qPxWBgTmLdOqSu9S1UbUx/Allow-customers-that-purchased-via-Reseller-to-access-Customers-Portal?node-id=2%3A29&t=ssRQtSMzrW04gs3t-1)

#### Implementation plan

| Step | Issue | MR/state |
|------|-------|-----------------------------|
| Create a `updated_namespace_change_flow` feature flag | https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5015 | ~"workflow::complete"  |
| Add a new component under the FF | https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5015 | https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/6894 ~"workflow::complete"  |
| Adjust the flow to work only for reseller customers | https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5015 | https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/6980 https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/7181 ~"workflow::complete" |
| Rollout FF (staging) | https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/6052 | TBD |
| Make sure the related docs are updated | https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5006 | :construction:  |
| Rollout FF (production) | https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/6052 | TBD |
| Clean up FF | https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/6052 | :one: |

## Result

Customer knows how to proceed should they run into issues associating purchased subscription to a namespace.

## Next steps (if any)

## How will we measure success?

Story points: 3.0