[Data] Deprecate prefetch_batches arg of iter_rows and change default value

[bveeramani]

Why are these changes needed?

This PR deprecates the prefetch_batches parameter of Dataset.iter_rows because it leaks an implementation detail (namely, that we use Dataset.iter_batches under-the-hood). This PR also increases the default value for 1) consistency with iter_batches and 2) to improve performance.

For historical context, #43349 erroneously introduced the prefetch_batches parameter.

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