Refactor GitLab Cells related configuration

Cells configuration in the Rails Application should look something like this:

  cell:
    enabled: boolean
    id: null
    database:
      skip_sequence_alteration: false
    topology_service:
      address: topology-service.gitlab.example.com:443
      ca_file: /home/git/gitlab/config/topology-service-ca.pem
      certificate_file: /home/git/gitlab/config/topology-service-cert.pem
      private_key_file: /home/git/gitlab/config/topology-service-key.pem

This would opening the current MRs:

• GitLab Rails application: !181708 (merged)
• GitLab Development Kit. See here Update: When we merged this, it broke old GitLab branches. So we are postponing this until %18.1
• Removing support of old Cells config format here: Something like gitlab-development-kit!4382 (diffs) (reverted MR) and !179385 (closed)

No default values are going to be changed

1. GitLab by default will keep the Cell disabled (cell.id is NULL and Topology Service is disabled).
2. GDK will have Cells enabled by default. Users can still disable it for Primary Cell: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/cells.md?ref_type=heads#disabling-the-http-router