E2E: New Subscriptions Page data-testid Migration

What does this MR do and why?

Relates to gitlab-org/quality/quality-engineering/team-tasks#1931 (closed) and gitlab-org/quality/quality-engineering/team-tasks#1735

In order to better unify our E2E testing framework with lower level tests and enable developers to more easily contribute to E2E tests, this MR migrates the data-qa-selectors to data-testids for the Gitlab::Page::Subscriptions::New page.

Please also note these conventions we will be following, from gitlab-org/quality/quality-engineering/team-tasks#1822 (closed):

• Use kebab-case for testid testid="kebab-case-attribute"
• Use strings for page elements - element :push_the_button becomes element 'push-the-button'

⚠ Important: ⚠ This relies on https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/8246 being merged first

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.