Prevent duplicate pipeline IIDs -- Next step: BBM to migrate existing iids to ci_pipeline_iids

Background

We currently have the unique index (project_id, iid, partition_id) on the table p_ci_pipelines. This ensures uniqueness within the same partition but not across partitions. Due to a possible workflow bug/race condition, we occasionally save duplicate pipeline iids for the same project, which causes problems such as https://gitlab.com/gitlab-com/request-for-help/-/issues/2855#note_2518831537.

We've explored a couple different possible solutions (https://gitlab.com/gitlab-org/gitlab/-/issues/545167#note_2802709422, https://gitlab.com/gitlab-org/gitlab/-/issues/545167#note_2844491364), and we ultimately decided that the only way to guarantee Pipeline iid uniqueness on the database level is to create a new table to track the iids. (See proposed approach in !210789 (comment 2869213142).)

In !213065 (merged), we introduced the static hash partitioned table p_ci_pipeline_iids. It has 64 partitions with the hash on project_id and primary key on (project_id, iid).
In !213457 (merged), we added database triggers on p_ci_pipelines so that it keep track of iids in p_ci_pipeline_iids.
In !213992 (merged), we are updating the internal ID initial_value logic to read maximum(:iid) from ci_pipeline_iids instead of ci_pipelines.

This issue is intended to track the next task in our implementation steps.

Create a BBM to migrate existing iids from p_ci_pipelines to p_ci_pipeline_iids.