Change default contempt from 21 to 24 centipawns

To top the rating lists and get more interesting middle play, it
is a good habit to set the default contempt to the highest value
that does not regress against contempt=0. We recently decreased
PawnValueEg it is logical that to raise a little bit the default
higher contempt because of the following internal dependency in
line 334 of search.cpp :

````
int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
````

STC: contempt=24 passed non-regression vs contempt=0
http://tests.stockfishchess.org/tests/view/5bd6d7f80ebc595e0ae21e14

LTC: contempt=24 passed non-regression LTC vs contempt=0
http://tests.stockfishchess.org/tests/view/5bd6e0980ebc595e0ae21f07

On 2018-11-01, we also tested the effects of contempt=21 and contempt=24
against Stockfish 9, and the net result was neutral:

Contempt 21
ELO: 51.68 +-1.9 (95%) LOS: 100.0%
Total: 40000 W: 9487 L: 3581 D: 26932
http://tests.stockfishchess.org/tests/view/5bdb1a140ebc595e0ae2620a

Contempt 24
ELO: 52.21 +-2.0 (95%) LOS: 100.0%
Total: 40000 W: 9759 L: 3793 D: 26448
http://tests.stockfishchess.org/tests/view/5bdb1b680ebc595e0ae2620d

Bench: 3459874