Migrate secrets to credentials

[jcraigk]

This moves the secret_key_base from secrets.yml to credentials to address the following deprecation warning when using load_defaults 7.1:

DEPRECATION WARNING: `Rails.application.secrets` is deprecated in favor of `Rails.application.credentials` and will be removed in Rails 7.2. (called from <top (required)> at /Users/jcraigk/code/phishin/config/environment.rb:2)

I'm hoping this will also fix production Devise logins, which appear to be broken currently (not able to reproduce locally). Disabling load_defaults 7.1 seems to fix the login issue, which might be a better solution until Devise addresses heartcombo/devise#5644