[CDF-26138]👷Change view syntax for cdf purge instances (#2150)

# Description

See below

## Changelog

- [x] Patch
- [ ] Skip

## cdf

### Changed

- [alpha] The command `cdf data purge instances` now expects the view to
be specified as a single string with the format
`space:externalId/version` instead of the list format `space externalId
version`. This is to be consistent with how views are specified in the
`cdf migrate commands`.

## templates

No changes.
