Curved lines at top of pipeline graph

Merged Annabel Dunstone Gray requested to merge 21126-pipeline-graph-updates into master

What does this MR do?

Adds curved lines to second-child of each column

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

To match the mockup

What are the relevant issue numbers?

Closes #21126 (closed)

Screenshots (if relevant)