Rename `trigger` to `downstream`
Problem to solve
ux-research#150 (comment 149620437) concluded on upstream
for https://gitlab.com/gitlab-org/gitlab-ee/issues/9045. Now the feature introduced by https://gitlab.com/gitlab-org/gitlab-ce/issues/52187 is using an inconsistent/outdated keyword.
Intended users
Further details
Proposal
The keyword trigger
should be updated accordingly with downstream