GH-50227: [Docs][Python][Parquet] Update parquet default version text in parquet.rst (#50228)

### Rationale for this change
See #50227 - Python Parquet section [Storing timestamps](https://arrow.apache.org/docs/dev/python/parquet.html#storing-timestamps) mentions version 1.0 is the default write version but the actual [default has been 2.6 ](https://github.com/apache/arrow/blame/98ee71ec2be85d3ac535be367c6edacc0bc09c7c/python/pyarrow/parquet/core.py#L1996) for some time.
(Replace old pr #37227)

### What changes are included in this PR?
Update the default Parquet version text 1.0 -> 2.6 in `docs/source/python/parquet.rst`.

### Are these changes tested?
Docs build fine locally.

### Are there any user-facing changes?
No.
* GitHub Issue: #50227

Authored-by: Tadeja Kadunc <tadeja.kadunc@gmail.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>

Author: tadeja

docs/source/python/parquet.rst

@@ -368,9 +368,9 @@ Storing timestamps
 
 Some Parquet readers may only support timestamps stored in millisecond
 (``'ms'``) or microsecond (``'us'``) resolution. Since pandas uses nanoseconds
-to represent timestamps, this can occasionally be a nuisance. By default
-(when writing version 1.0 Parquet files), the nanoseconds will be cast to
-microseconds ('us').
+to represent timestamps, this can occasionally be a nuisance. When writing
+older ``version='1.0'`` or ``version='2.4'`` Parquet files, the nanoseconds
+will be cast to microseconds (``'us'``).
 
 In addition, We provide the ``coerce_timestamps`` option to allow you to select
 the desired resolution:
@@ -388,17 +388,16 @@ an exception will be raised. This can be suppressed by passing
    >>> pq.write_table(table, 'example.parquet', coerce_timestamps='ms',
    ...                allow_truncated_timestamps=True)
 
-Timestamps with nanoseconds can be stored without casting when using the
-more recent Parquet format version 2.6:
+Timestamps with nanoseconds can be stored without casting when using
+Parquet format version 2.6, which is the default:
 
 .. code-block:: python
 
    >>> pq.write_table(table, 'example.parquet', version='2.6')
 
-However, many Parquet readers do not yet support this newer format version, and
-therefore the default is to write version 1.0 files. When compatibility across
-different processing frameworks is required, it is recommended to use the
-default version 1.0.
+However, some Parquet readers may not yet support this newer format version.
+When compatibility across different processing frameworks is required, the
+older ``version='1.0'`` or ``version='2.4'`` remain available.
 
 Older Parquet implementations use ``INT96`` based storage of
 timestamps, but this is now deprecated. This includes some older