[Cells 1.0] (Size: S) Decide what do to with `db_key_base`

Problem

Decide what to do with this secret with respect to Cells

If we have different db_key_base for different cells:

• Organizations migrating to different cells will have corrupted encrypted values. Or we need to re-encrypt the data.
• Syncing clusterwide data like application_settings#runners_registration_token will not work either.

Proposal (2024-09-17)

We use the same db_key_base on all Cells. Increasing the infrastructure security of GitLab.com isn't listed a goals of Cells anyway.

Once we'll exclusively use ActiveRecord::Encryption, we'll be able to rotate secrets and re-encrypts data that were encrypted with a legacy key, see &15226 for the possible workflow. The rotation support is out of scope for Cells 1.0.