[E2E] PROMOTE TO BLOCKING: trigger_email_notification_spec.rb

What does this MR do?

Promotes the following e2e tests to the blocking bucket:

1. Plan Email Notification is received by a user for project invitation | Testcase | Spec metrics

This MR was created based on data from reliable e2e test report: #472396 (closed)

(This MR was automatically generated by gitlab_quality-test_tooling at 2024-07-15 02:07:38 UTC)