Clean up tmp FKs p_ci_stages

Ref: #439065 (closed)

What does this MR do and why?

Following up !146459 (merged) where we validated this FK for self-managed and add it to our routing table.

This MR renames our temporary FK and drop the old one to make sure it points to the new the routing table p_ci_stages.

Why are we doing this?

Remaining step to clean make sure the routing table matches the original table that we partition.

https://docs.gitlab.com/ee/development/database/partitioning/list.html#step-7---re-point-foreign-keys-to-parent-table