Scaffold new billings upgrade subscription page

What does this MR do and why?

Scaffold new billings upgrade subscription page

• Create new feature flag
• Add new route / page
• Add new vue app

This page is gated by the introduced FF while in development. The FF is defaulted to off.

There are not any current entry points to this page (other than direct url entry)

Changelog: added

EE: true

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start GDK in Saas GITLAB_SIMULATE_SAAS: 1
2. Turn on the feature flag:
   1. http://127.0.0.1:3000/rails/features/
   2. Enable saas_upgrade_subscription_page
3. Navigate to the new page:
   1. http://127.0.0.1:3000/groups/[group-name]/-/billings/upgrade_subscription
   2. If you have Vue Dev tools, open up the app in your browser console and verify the injected props (shown in above screenshot)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #593611