[Testing] Preview subscription update workflow using Orders API

### Problem

This issue is to track the verification process to enable the use of 'Orders API in Subscription Upgrade workflow'.

**Associated FF:** [`use_orders_api_to_preview_amendment`](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4774)

### Testing scenarios
| Scenario | Steps | Expected result | Pass? (Staging environment) | Notes |
|----------|-------|-----------------|-----------------------------|-------|
| 1. Purchase additional seats | 1. Go to manage purchases <br> 2. Choose an active subscription or create one <br> 3. Click on Add more seats | Values match when ff is on and off | :white_check_mark: Verified. https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5063#note_1207670522. |  |
| 2. CI minutes purchase from customers app |  1. Go to manage purchases <br> 2. Choose an active subscription or create one <br> 3. Click on Buy more CI minutes | Values match when ff is on and off |  :white_check_mark: Verified. https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5063#note_1207693427 |  |
| ~CI minutes purchase from Gitlab app~ |  |  |  | N/A. Doesn't use amendments, fetches plans data via graphql. |
| 3. Storage purchase from customers app | 1. Go to manage purchases <br> 2. Choose an active subscription or create one <br> 3. Click on Buy more storage | Values match when ff is on and off | :white_check_mark: Verified. https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5063#note_1207697540 |  |
| ~Storage purchase from Gitlab app~ |  |  |  | N/A. Covered in preview graphql scenario|
| 4. Renew without upgrade | 1. Go to manage purchases <br> 2. Choose an active subscription thats up for renewal or create a subscription with activation date in the past via the API <br> 3. Click on Renew | Values match when ff is on and off | :white_check_mark: Verified. https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5063#note_1208808134 |  |
| 5. Renew with upgrade | 1. Go to manage purchases <br> 2. Choose an active subscription thats up for renewal or create a subscription with activation date in the past via the API <br> 3. Click on Renew <br> 4. On renewal page change plan to Ultimate | Values match when ff is on and off | :white_check_mark: Verified. https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5063#note_1208810015 |  |
| 6. Renew with purchase of true ups | 1. Go to manage purchases <br> 2. Choose an active subscription thats up for renewal or create a subscription with activation date in the past via the API <br> 3. Click on Renew <br> 4. On renewal page add additional seats | Values match when ff is on and off | :white_check_mark: Verified. https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5063#note_1208813697 |  |
| 7. Renew with purchase of true ups and upgrade | 1. Go to manage purchases <br> 2. Choose an active subscription thats up for renewal or create a subscription with activation date in the past via the API <br> 3. Click on Renew <br> 4. On renewal page add additional seats and upgrade option | Values match when ff is on and off | :white_check_mark: Verified. https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5063#note_1208814642 |  |
| 8. Change namespace to group with insufficient seats | 1. Go to manage purchases <br> 2. Choose an active gitlab.com subscription <br> 3. Click on Change namespace <br> 4. In the change namespace screen choose a group that is using more seats than the current one | Values match when ff is on and off | :white_check_mark: Verified. https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/5063#note_1208815854 |  |

### Result

### Next steps (if any)

### How will we measure success?

Story points: 2.0