Prepare partitioning constraint for ci_stages

Ref: #428988 (closed)

What does this MR do and why?

Following up !144167 (merged) where we successfully added this constraint for gitlab.com.

This MR adds a migration to create this new constraint for self-managed users.

This is a similar approach we used for ci_job_artifacts in !144167 (merged).

Why are we doing this?

This is a prerequisite in order to partition this table.

https://docs.gitlab.com/ee/development/database/partitioning/list.html#step-6---create-parent-table-and-attach-existing-table-as-the-initial-partition