Rename pipeline service from "cross-project" to "downstream"

What does this MR do?

This MR is a follow-up from !20930 (merged) where we added the ability to create a downstream pipeline within the same project of the upstream pipeline.

By making this change, the class name is no longer creating only cross-project pipelines. I've decided to use a slightly more generic name that includes both use cases.

This MR must be merged after: !20930 (merged)

Related to: #16094 (closed) (primary issue)