APCu: fix return value of setMultiple()

[davidrans]

Problem

Backend::setMultiple() has a bool return type, but the child class's implementation, APCu::setMultiple(), returns an array. We should address the inconsistency.

Background

https://github.com/iFixit/Matryoshka/blob/master/library/iFixit/Matryoshka/Backend.php#L37
https://github.com/iFixit/Matryoshka/blob/master/library/iFixit/Matryoshka/APCu.php#L16
https://www.php.net/manual/en/function.apcu-store.php#refsect1-function.apcu-store-returnvalues