[E2E] PROMOTE TO BLOCKING: rebase_merge_request_spec.rb

What does this MR do?

Promotes the following e2e tests to the blocking bucket:

1. Create Merge request rebasing user rebases source branch of merge request | Testcase | Spec metrics

This MR was created based on data from reliable e2e test report: #454762 (closed)

(This MR was automatically generated by gitlab_quality-test_tooling at 2024-04-08 02:05:29 UTC)