Refactor to use public API `Series.str.find_multiple`in query 18

[bdice]

I noticed that gpu-bdb query 18 was using private methods from cudf for find_multiple:

https://github.com/rapidsai/gpu-bdb/blob/f48c05d63d5cb4baa59708cb262506f6d9d3f4f1/gpu_bdb/bdb_tools/q18_utils.py#L21

https://github.com/rapidsai/gpu-bdb/blob/f48c05d63d5cb4baa59708cb262506f6d9d3f4f1/gpu_bdb/bdb_tools/q18_utils.py#L127

cudf now has public APIs that perform the same task: Series.str.find_multiple. This should be refactored to use public APIs.

See also: https://github.com/rapidsai/cudf/issues/10126#issuecomment-1021711141