[Cells 1.0] Migrate from `Rails.application.secrets` to `Rails.application.credentials`

Rails.application.secrets is deprecated since Rails 5.2 (see https://www.shakacode.com/blog/rails-7-1-removes-secret-setup-command-and-deprecates-secret-show-edit-commands/).

Also see https://guides.rubyonrails.org/security.html#custom-credentials and previous work at: #26243 (comment 1474177720).

Estimate

2 backend days.