[E2E] PROMOTE TO BLOCKING: user_index_spec.rb

What does this MR do?

Promotes the following e2e tests to the blocking bucket:

1. Data Stores When using advanced search API to search for a user finds the user that matches username | Testcase | Spec metrics

This MR was created based on data from reliable e2e test report: #473634

(This MR was automatically generated by gitlab_quality-test_tooling at 2024-07-22 02:05:52 UTC)