Fix when MSRV>=1.93

[larseggert]

We need rust 1.93...

        while self.used > target { // note renamed from reduce
            let Some(e) = self.dynamic.pop_back_if(|e| e.can_reduce(self.acked_inserts_cnt)) else {
                return false;
            };
            self.used -= u64::try_from(e.size()).expect("usize fits in u64");
        }

As it is, I think that what you have is fine. It's just not as idiomatic as you might like.

Originally posted by @martinthomson in #3181 (comment)