This change makes b-tree sets conform to the std::set API of having const access through `iterator`s as well as `const_iterator`s.

This change can cause breakages for user code that depends on having mutable access to keys. If your code breaks, then there a couple of options to fix the issue:
- If you are mutating a part of the key that can impact the sorted order, then this is a bug and you need to extract the key, mutate it, and then re-insert the mutated key.
- If you are mutating a part of the key that can't impact the sorted order, then you can potentially (a) refactor to use btree_map instead of btree_set and make the part of the key that doesn't impact that ordering be the mapped_type, (b) change the part of the key that doesn't impact the ordering to be a `mutable` member of the key_type, (c) refactor from btree_set<K> to btree_set<K*> (but this also permits mutable access to the part of the key that determines the ordering).

PiperOrigin-RevId: 563118156
Change-Id: I243558e74c43aa6655290099494b411d15298f4c