[E2E] PROMOTE TO BLOCKING: log_into_mattermost_via_gitlab_spec.rb

What does this MR do?

Promotes the following e2e tests to the blocking bucket:

1. Govern Mattermost login user logs into Mattermost using GitLab OAuth | Testcase | Spec metrics

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

(This MR was automatically generated by gitlab_quality-test_tooling at 2024-07-22 02:06:07 UTC)