E2E: Fix Stale Subscription Header Selector

What does this MR do and why?

Fixes #422007 (closed)

Updates the subscription_header selector to an h5 element introduced in !129072 (merged)

Note: The remainder of qa/lib/gitlab/page/group/settings/billing.rb will be updated to use data-testids in gitlab-org/quality/quality-engineering/team-tasks#1931 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

This test can only run against Staging. You can test out the QA changes only by running this spec against Staging locally:

1. cd into qa directory
2. Run the following command

GITLAB_USERNAME=gitlab-qa GITLAB_PASSWORD=<get from 1Password> GITLAB_QA_USER_AGENT=<get from 1Password> GITLAB_ADMIN_USERNAME=gitlab-qa-bot GITLAB_ADMIN_PASSWORD=<get from 1Password> GITLAB_QA_ACCESS_TOKEN=<get from 1Password> GITLAB_QA_ADMIN_ACCESS_TOKEN<get from 1Password> QA_GITLAB_URL=https://staging.gitlab.com bundle exec rspec qa/specs/features/ee/browser_ui/11_fulfillment/utilization/billing_seats_usage_data_spec.rb

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.