Set `gitlab_schema: gitlab_shared` for Batched Background Migration tracking tables

Extracted from !84764 (comment 910045400) all tables related to batched background migrations need to be changed from gitlab_main to gitlab_shared in https://gitlab.com/gitlab-org/gitlab/-/blob/f9af2df05b362ba1b58b07d0d85d922926b7a720/lib/gitlab/database/gitlab_schemas.yml#L45 . This is necessary to support CI decomposition as without this we won't be able to write batched background migrations that iterate over CI tables.