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GH-36882: [C++][Parquet] Default RLE for bool values in the parquet version 2.x (#36955)

### Rationale for this change

RLE is usually more efficient than PLAIN encoding for boolean columns, and it is already enabled by default in parquet-mr and arrow-rs.

### What changes are included in this PR?

* Slight breaking change in ColumnProperties to set default encoding to UNKNOWN (used to be PLAIN).
* If UNKNOWN is given, let the column writer decide the column encoding according to the selected Parquet format version and the column type.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes, default encoding of boolean type has been switched to RLE when the selected Parquet format version is at least 2.0 (the current default version is 2.6). It used to always be PLAIN.

* Closes: #36882

Authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>