[prep CDF-26068] 🐛 Fix migration model (#2126)

# Description

Rename the property `classicExternalId` to `dataSetExternalId` as it is
much more descriptive. In addition, adding an index for fast lookup on
this property.

## Changelog

- [x] Patch
- [ ] Skip

## cdf

### Fixed

- [alpha] Change the data model deployed with `cdf migrate prepare`.

## templates

No changes.
