Subscription Activation Modal: Better arrange GraphQL mutation and query

What does this MR do?

This MR is part of a larger work towards implementing the feature described in gitlab-org/gitlab#328246.

The failing scenarios are handled in the context of this issue and, for the modal, of the following ones.

Use the mutation data to update the current subscription	👈 you are here
Create the modal and button, reusing the Activation Form	tbd
Plug the previous 2	tbd
Write feature specs	tbd

A feature flag is not used because this feature is for self-managed instances. The feature is behind an Application Setting and it's not released nor documented. To test it locally, see the following steps.

To test the changes locally

1. Delete any previous license you might have

2. Make sure you have set the following environment vars (usually in your terminal) and restart gdk

   • GITLAB_LICENSE_MODE is set to test
   • CUSTOMER_PORTAL_URL is set to https://customers.stg.gitlab.com

3. Use Rails console to run ApplicationSetting.current.update(cloud_license_enabled: true)

4. Navigate to admin/cloud_license

5. Use the following activation code: Lx6XhhLR1jSj2aacuZFRP8Mf

6. To test the failing scenario, simply block the network request to sync_seat_link

Screenshots (strongly suggested)

This MR contains no visual changes.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because the changes have no UI impact.
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team