Lower smallnet threshold linearly as pawn count decreases

Lower smallnet threshold linearly as pawn count decreases

Passed STC:
https://tests.stockfishchess.org/tests/view/6644f677324e96f42f89d894
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 377920 W: 97135 L: 96322 D: 184463
Ptnml(0-2): 1044, 44259, 97588, 44978, 1091

Passed LTC:
https://tests.stockfishchess.org/tests/view/664548af93ce6da3e93b31b3
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 169056 W: 42901 L: 42312 D: 83843
Ptnml(0-2): 58, 18538, 46753, 19115, 64

closes https://github.com/official-stockfish/Stockfish/pull/5252

Bench: 1991750