Cells: Iteration 2: Convert FK between `routes` and `namespaces`

See epic: &11143 (closed) for details.

We need to convert the FK in routes.namespace_id referencing namespaces table to loose foreign key.

Failing spec for reference: https://gitlab.com/gitlab-org/gitlab/-/jobs/4796612578

Steps

1. From db/gitlab_schemas/gitlab_main_cell.yaml
2. Remove: allow_cross_foreign_keys: gitlab_main_clusterwide.
3. Pipeline will fail
4. Convert the FK to loose foreign key
5. Get pipeline green.