Promotes pipelines_for_merged_results_and_merge_trains_spec to reliable

What does this MR do?

Related to https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/980

Promotes qa/qa/specs/features/ee/browser_ui/4_verify/pipelines_for_merged_results_and_merge_trains_spec.rb to :reliable that will block the deployment upon failure.

Does this MR meet the acceptance criteria?

Conformity

• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.