[ADT] Relax iterator constraints on all_equal (#106400)
The previous `all_equal` implementation contained `Begin + 1`, which
implicitly requires `Begin` to model the
[random_access_iterator](https://en.cppreference.com/w/cpp/iterator/random_access_iterator)
concept due to the usage of the `+` operator. By swapping this out with
`std::next`, this method can be used with weaker iterator concepts, such
as
[forward_iterator](https://en.cppreference.com/w/cpp/iterator/forward_iterator).
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Co-authored-by: Jakub Kuderski <kubakuderski@gmail.com>
