32708.Fix - Scikit Learn

Both :class:ensemble.GradientBoostingRegressor and :class:ensemble.GradientBoostingClassifier with the default "friedman_mse" criterion were computing impurity values with an incorrect scaling, leading to unexpected trees in some cases. The implementation now uses "squared_error", which is exactly equivalent to "friedman_mse" up to floating-point error discrepancies but computes correct impurity values. By :user:Arthur Lacote <cakedev0>.