Simplify score improvement reduction

Simplify score improvement reduction

Reduce depth by 2 based on score improvement, only for depths 3 to 11.

Simplification STC: https://tests.stockfishchess.org/tests/view/64929a53dc7002ce609c7807
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 238912 W: 63466 L: 63468 D: 111978
Ptnml(0-2): 564, 26262, 65805, 26262, 563

Simplification LTC: https://tests.stockfishchess.org/tests/view/64942e47dc7002ce609c9e07
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 64452 W: 17485 L: 17320 D: 29647
Ptnml(0-2): 19, 6161, 19706, 6316, 24

closes https://github.com/official-stockfish/Stockfish/pull/4637

Bench: 2740142