Downstream pipeline nodes are currently not visually connected correctly

Problem to solve

Currently, downstream are represented by nodes at the far right of a pipeline graph regardless of which stage and job did trigger them. This creates a visually incorrect state which does not accurately represent reality.

https://gitlab.com/gitlab-org/gitlab-ee/pipelines/34943331

Further details

The current state will become more complicated when looking at https://gitlab.com/gitlab-org/gitlab-ce/issues/47063

This came up when discussing https://gitlab.com/gitlab-org/gitlab-ce/issues/52187 and https://gitlab.com/gitlab-org/gitlab-ce/issues/39640

Proposal

Remove connecting lines from current pipeline stages towards downstream pipeline nodes as they are not visually correct. However, maintain a visual stage independent relationship in one way or another between downstream pipeline node and the pipeline it comes from.

Also, consider the effects on upstream pipeline node connections.

If possible, https://gitlab.com/gitlab-org/gitlab-ce/issues/54331 should not be blocked by this issue.

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

The pipeline graph accurately represents reality in regards to jobs triggering downstream pipelines

Links / references

cc: @grzesiek