refactor!: remove deprecated APIs (#799)

## Summary

Removes deprecated APIs as part of the v3 major release:

- `DatasetClient.download_items()` — deprecated alias of
`get_items_as_bytes()`.
- `max_unprocessed_requests_retries` and
`min_delay_between_unprocessed_requests_retries` arguments of
`batch_add_requests` (sync + async). The docstrings already said *"Will
be removed in next major release."*
- `exclusive_start_id` argument of `list_requests` (sync + async) —
replaced by `cursor`.

Also drops the now-unused `import warnings` from both resource client
modules and the associated unit tests for the removed
`cursor`/`exclusive_start_id` mutex check.