Tweak reduction formula based on depth

The idea came to me by checking for trends from the megafauzi tunes, since the values of the divisor for this specific formula were as follows:
stc: 15990
mtc: 16117
ltc: 14805
vltc: 12719
new vltc passed by Muzhen: 12076

This shows a clear trend related to time control, the higher it is, the lower the optimum value for the divisor seems to be.
So I tried a simple formula, using educated guesses based on some calculations, tests show it works pretty fine, and it can still be further tuned at VLTC in the future to scale even better.

Passed STC:
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 431360 W: 110791 L: 109898 D: 210671
Ptnml(0-2): 1182, 50846, 110698, 51805, 1149
https://tests.stockfishchess.org/tests/view/663770409819650825aa269f

Passed LTC:
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 114114 W: 29109 L: 28625 D: 56380
Ptnml(0-2): 105, 12628, 31101, 13124, 99
https://tests.stockfishchess.org/tests/view/66378c099819650825aa73f6

https://github.com/official-stockfish/Stockfish/pull/5223

bench: 2273551

src/search.cpp

@@ -1154,7 +1154,7 @@ moves_loop:  // When in check, search starts here
                       + (*contHist[3])[movedPiece][move.to_sq()] - 5078;
 
         // Decrease/increase reduction for moves with a good/bad history (~8 Elo)
-        r -= ss->statScore / 12076;
+        r -= ss->statScore / std::max(21000 - (depth * 305), 12000);
 
         // Step 17. Late moves reduction / extension (LMR, ~117 Elo)
         if (depth >= 2 && moveCount > 1 + rootNode)