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 Now the feature introduced by https://gitlab.com/gitlab-org/gitlab-ce/issues/52187 is using an inconsistent/outdated keyword.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

Permissions and Security

Documentation

What does success look like, and how can we measure that?

What is the type of buyer?

Links / references