Ensure uniqueness ci_pipeline_variables

Ref: #429939 (closed)

What does this MR do and why?

This MR ensures uniqueness of ci_pipeline_variables across other partitions now that we successfully created its partition table.

It leverages the recent helper introduced in !138739 (merged) allowing use to create the trigger only on the routing table now that we are running PG14 in production.

https://docs.gitlab.com/ee/development/database/partitioning/list.html#step-8---ensure-id-uniqueness-across-partitions