Add overage modal E2E spec

What does this MR do and why?

Partly addresses customers-gitlab-com#5047 (closed). Adds an E2E spec to test overage modal feature.

Will open another MR for when subscription is premium.

How to set up and validate locally

Can run only against staging.gitlab.com

GITLAB_ADMIN_USERNAME=XXXX GITLAB_ADMIN_PASSWORD="xxx" GITLAB_QA_ACCESS_TOKEN=XXXX GITLAB_USERNAME=gitlab-qa GITLAB_PASSWORD="XXXX" GITLAB_QA_USER_AGENT="XXXXX" WEBDRIVER_HEADLESS=false QA_DEBUG=true QA_GITLAB_URL=https://staging.gitlab.com bundle exec rspec ./qa/specs/features/ee/browser_ui/11_fulfillment/utilization/overage_modal_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.