[VETS-API] Fix Deprecation Warnings ahead of Rails 7.2

[ryan-mcneil]

There are a handful of DEPRECATION WARNINGs that need to be addressed prior to upgrading Rails to 7.2 (and ✨8✨ )

Tasks

• Fix the following:
  • DEPRECATION WARNING: config.active_support.remove_deprecated_time_with_zone_name is deprecated and will be removed in Rails 7.2. (in specs)
  • DEPRECATION WARNING: Your secret_key_baseis configured inRails.application.secrets, which is deprecated in favor of Rails.application.credentials and will be removed in Rails 7.2. (in specs)
  • DEPRECATION WARNING: Using preview_path= option is deprecated and will be removed in Rails 7.2. Please use preview_paths= instead. (local server)

Acceptance Criteria

• Deprecation warnings blocking Rails upgrade are all addressed