[ENH] reduce private coupling of IndividualBOSS classifier and BaseClassifier

[fkiraly]

This PR removes an instance of problematic private coupling of IndividualBOSS classifier to the boilerplate layer of BaseClassifier, which could make IndividualBOSS and dependents break in boilerplate refactor such as #5408.

The _shorten_bags logic internally made a "fitted clone" of self, with explicit copying of attributes, that assumed specifics of the private structure of BaseClassifier such as "names of all private attributes" that were hard-coded in IndividualBOSS.

This coupling has been removed to the largest extent by instead using deepcopy and changing only the attributes that need to change.