When is it necessary to use StandardScaler/MinMaxScaler on y_train and y_test?
As far as I know, scaling target values is not the common practice, though I do not expect it to change results much. Here is a more detailed discussion of this issue:

https://stats.stackexchange.com/questions/111467/is-it-necessary-to-scale-the-target-value-in-addition-to-scaling-features-for-re