arrow

GH-36537: [Python] Ensure dataset writer follows default Parquet version of 2.6 (#36538)

### Rationale for this change

When bumping the default Parquet write version from 1.0 to 2.4 and then to 2.6, we forgot to also bump that default in the parquet.dataset writer (`ParquetFileWriteOptions`).

This PR bumps that directly from 1.0 to 2.6 to follow the default of the pyarrow.parquet module.

### Are these changes tested?

Yes

### Are there any user-facing changes?

Different default version can give different types in the parquet file.

* Closes: #36537

Authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>