Edits to use 'merged results' for consistency

app/assets/javascripts/vue_merge_request_widget/constants.js

app/presenters/ci/pipeline_presenter.rb

doc/ci/pipelines/downstream_pipelines.md

doc/ci/pipelines/merge_trains.md

doc/ci/pipelines/mr_pipeline_troubleshooting.md

locale/gitlab.pot

...ee/browser_ui/4_verify/pipeline_for_merged_result_spec.rb

spec/features/merge_request/user_sees_merge_widget_spec.rb

spec/presenters/ci/pipeline_presenter_spec.rb

spec/serializers/ci/pipeline_entity_spec.rb

spec/serializers/merge_requests/pipeline_entity_spec.rb