Do more aggressive probCut when improving.
If improving is true reduce depth of probCut search by 1.
Passed STC:
https://tests.stockfishchess.org/tests/view/6a2a6ceb17167cbe7100a909
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 119136 W: 30591 L: 30158 D: 58387
Ptnml(0-2): 323, 13831, 30845, 14228, 341
Passed LTC:
https://tests.stockfishchess.org/tests/view/6a2d65a40d5d4b19d080537f
LLR: 2.96 (-2.94,2.94) <0.50,2.50>
Total: 186552 W: 47377 L: 46755 D: 92420
Ptnml(0-2): 89, 20047, 52395, 20643, 102
closes https://github.com/official-stockfish/Stockfish/pull/6914
Bench: 2422062