BUG: avoid specifying default coerce_timestamps in to_parquet
Looking into the usage question of https://github.com/pandas-dev/pandas/issues/31572, I noticed that specifying the version to allow writing nanoseconds to parquet worked in plain pyarrow code, but not with pandas' `to_parquet`. 
This is because we hardcode `coerce_timestamps="ms"` while the default is `None`, which has version-dependent behaviour (eg if version="2.0", actually write the nanosecond data)