[DEV-14868] DEFCGold -> CSVModel Fix

[dpb-bah]

Description:

• Updating several models (migrations, external_load_date) to Gold
• Updating DEFCSilver(DeltaModel) -> DEFCGold(CSVModel) including some script names

Technical Details:

• Includes a minor fix for the CSVModel to preserve arrays between saving and loading from pandas

Requirements for PR Merge:

1. Unit & integration tests updated
2. Documentation updated
3. Data validation completed (examples listed below)
   1. Is performance impacted in the changes (e.g., API, pipeline, downloads, etc.)?
   2. Is the expected data returned with the expected format?
4. Appropriate Operations ticket(s) created
5. Jira Ticket(s)
   1. DEV-14668
6. Developer Reviews
   • Broker Developer Review
   • USAS Developer Review
7. Merged before Merge Broker External and Submissions Buckets #4

Explain N/A in above checklist: