Unquarantine mattermost login spec

What does this MR do?

Unquarantine log_into_mattermost_via_gitlab_spec.rb.

Test failed because of the #202079 (closed), which was fixed with !24498 (merged).

Closes #202069 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content