[Cells 1.0] Decide what to do with `secrets.ci_jwt_signing_key`

Decide what to do with this secret with respect to Cells.

Options:

• Remove this secret as it is un-used
• Per cell
• Ensure it is the same for all cells
• Something else

Note, https://docs.gitlab.com/ee/development/application_secrets.html#secret-entries says, but we actually intend to rename it in #428129:

The base key for encrypting the CI_JOB_JWT and CI_JOB_JWT_V2 predefined CI/CD variables. CI_JOB_JWT and CI_JOB_JWT_V2 were deprecated in GitLab 15.9 and are scheduled to be removed in GitLab 17.0.