…rade with `MERGED` pick_version (elastic#197262)

## Summary

The PR elastic#191439 enhanced the
`/upgrade/_perform` API contract and functionality to allow the users of
the endpoint to upgrade rules to their `MERGED` version.

However, a bug slipped in, where the two different types of `DataSource`
(`type: index_patterns` or `type: data_view_id`) weren't properly
handled and would cause, in some cases, a rule payload to be created
having both an `index` and `data_view` field, causing upgrade to fail.

This PR fixes the issue by handling these two field in a specific way,
checking what the `DataSource` diffable field's type is, and setting the
other field to `undefined`.

### Checklist

Delete any items that are not applicable to this PR.

- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#_add_your_labels)
- [ ] This will appear in the **Release Notes** and follow the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

(cherry picked from commit 9656621)