Enable `allow_single_file_parallelism` by default to write out parquet files in parallel

[alamb]

Is your feature request related to a problem or challenge?

In #7562 @devinjdangelo added the (really neat) feature to write a single parquet file in parallel

However it is not yet enabled by default because the index structures aren't populated correctly -- see #7589

Describe the solution you'd like

I would like to turn on allow_single_file_parallelism by default after the index structure issue is resolved

Describe alternatives you've considered

No response

Additional context

No response