deprecate intset to bitset

[Sacha0]

#24263 / #24281 reminded me that renaming IntSet to BitSet received reasonable support in #20512. This pull request provides that rename. Best!

[Sacha0]

Thanks all! :)

[rfourquet]

Personally I'm not conviced by this change, I don't really see the strong connection to bits. Sure it can be used to reference bits, but the interface of it is to be an AbstractSet which can store only positive integers: what is specific to bits in that? At least it would be useful to explain the connection to bits in the docstring. But I admit I know only bitset in C++, where this represents a fixed number of bits with a specific interface (different from sets), so I may miss the precedent from other languages. On a side note, I had an in-progress branch where I explored storing any Int in IntSet, but don't know yet if the overhead would be minimal enough for base...