[E2E] PROMOTE TO BLOCKING: custom_email_spec.rb

What does this MR do?

Promotes the following e2e tests to the blocking bucket:

1. Plan Custom email customizes email with additional text | Testcase | Spec metrics

This MR was created based on data from reliable e2e test report: #474783 (closed)

(This MR was automatically generated by gitlab_quality-test_tooling at 2024-07-29 02:06:39 UTC)