Saas user limit experience e2e test coverage

What does this MR do and why?

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/387656

Add coverage for scenarios from the issue above for Sass user limit experience feature

Feature flags for feature:

preview_free_user_cap
notification_free_user_cap_show_over_limit
free_user_cap
free_user_cap_new_namespaces

How to set up and validate locally

Runs only against Staging due to Trial requiring Zuora central sandbox

GITLAB_QA_USER_AGENT=xxx GITLAB_ADMIN_USERNAME=gitlab-qa-bot GITLAB_ADMIN_PASSWORD=xxx GITLAB_QA_ADMIN_ACCESS_TOKEN=xxx QA_GITLAB_URL=https://staging.gitlab.com WEBDRIVER_HEADLESS=false bundle exec bin/qa Test::Instance::All ./qa/specs/features/ee/browser_ui/11_fulfillment/saas_user_limit_experience_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.