[CDF-25389]  Update workflow version loader (#1813)

# Description

**Context:** Cognite Toolkit has standardized classes for all CDF
resources for all CRUD operations called loaders. This PR updates the
loader for Workflow and WorkflowVersion resources.

We added support for the CRUD operations for Workflow/Workflow Version
resources in Toolkit before there was a retrieve endpoint. As a
workaround, we used list. This PR takes advantage of the new retrieve
endpoint instead of the workaround as we have seen user that had trouble
with existing workflow versions not being detected as existing.

## Changelog

- [x] Patch
- [ ] Minor
- [ ] Skip

## cdf

### Fixed

- When deploying `WorkflowVersions`, Toolkit has a more robust way to
check whether the workflow version already exists.

## templates

No changes.
