Replace FKs for p_ci_stages

Ref: #439065 (closed)

What does this MR do and why?

This MR creates a FK pointing to p_ci_stages now that we partitioned Ci::Stage as it doesn't exist for the moment.

It creates the constraint and marks it as invalid as it will be validated over the weekend on gitlab.com.

Why are we doing this?

Now the we successfully partitioned ci_stages we need to make sure the referenced FK is targeting our new routing table.

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