Set Use NNUE by default to true

Since the initial stages of the merge, progress has been made so that
this seems the best option now:

* NNUE is clearly stronger on most relevant hardware and time controls
* All of our CI and testing infrastructure has been adjusted
* The default net is easy to get (further ideas #3030)

fixes https://github.com/official-stockfish/Stockfish/issues/2861

closes https://github.com/official-stockfish/Stockfish/pull/3033

No functional change.