[C++][Parquet] "Keep redacted metadata version for old readers"

### Describe the bug, including details regarding any error messages, version, and platform.

This code here looks slightly fishy:
https://github.com/apache/arrow/blob/8b469b179da3bda89d5e8d24c82523553f5aca5c/cpp/src/parquet/metadata.cc#L1787-L1796

Basically, when encrypting a Parquet file with plaintext footer, the Parquet writer _also_ keeps most of the column metadata in plaintext (except statistics).

### Component(s)

C++, Parquet

Read cpp/src/parquet/metadata.cc at lines 1787-1796 and trace the plaintext-footer encryption path. The report names no test and does not specify the expected metadata-version behavior, so clarify the old-reader compatibility requirement before changing code. Done means the intended behavior is agreed and verified by an appropriate Parquet test.

Written by the indexing model from the issue text.