Adjust aspiration window

Narrow the aspiration window after fail high.

Passed STC:
LLR: 2.98 (-2.94,2.94) <0.00,2.00>
Total: 51296 W: 13550 L: 13207 D: 24539
Ptnml(0-2): 165, 5971, 13052, 6276, 184
https://tests.stockfishchess.org/tests/view/68d99afffa806e2e8393b7ae

Passed LTC;
LLR: 2.94 (-2.94,2.94) <0.50,2.50>
Total: 87780 W: 22795 L: 22375 D: 42610
Ptnml(0-2): 52, 9340, 24694, 9744, 60
https://tests.stockfishchess.org/tests/view/68dae0a6fa806e2e8393baad

See the comments in #6293 discussing the mechanisms leading to issue #6296

closes https://github.com/official-stockfish/Stockfish/pull/6337

Bench: 2336606

AUTHORS

@@ -183,6 +183,7 @@ Nathan Rugg (nmrugg)
 Nguyen Pham (nguyenpham)
 Nicklas Persson (NicklasPersson)
 Nick Pelling (nickpelling)
+Nicolas Duhamel (nikloskoda)
 Niklas Fiekas (niklasf)
 Nikolay Kostov (NikolayIT)
 Norman Schmidt (FireFather)

src/search.cpp

@@ -381,6 +381,7 @@ void Search::Worker::iterative_deepening() {
                 }
                 else if (bestValue >= beta)
                 {
+                    alpha = std::max(beta - delta, alpha);
                     beta  = std::min(bestValue + delta, VALUE_INFINITE);
                     ++failedHighCnt;
                 }