Cells: Move `broadcast_messages` into `clusterwide`

What does this MR do and why?

This table is already configured from Admin Area and is cluster-wide. This uses a work already done for application_settings.

Since this is accessed in isolation no future work is required to split joins.

References:

• related to: Cells: Iteration 1: Introduce attach `appearanc... (#397697 - closed)
• similar to: Pods: Iteration 1: Introduce `gitlab_main_clust... (#388275 - closed)
• similar to: Pods: Iteration 1: Introduce `ClusterMain::Appl... (#388276 - closed)

Follow-ups:

• Cells: Fix `broadcast_messages` Redis cache (#397699)
• Cells: Iteration 1: Remove `broadcast_messages.... (#398151 - closed)

Foreign key removal

This change reverts parts of DB change introduced by !95913 (merged). We will rethink how to approach this at an organization-level. However, the usage of namespace_id will be incorrect regardless what we decide.

How to set up and validate locally

GDK: Follow the process from: !109439 (merged)

GCK: Use this branch: gitlab-compose-kit!233 (merged) and follow the environment configuration.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.