[oneDPL] Fix the function signatures to not require std::projected_value_t

[akukanov]

Another oversight found in the signatures of range algorithms: std::projected_value_t requires C++26.

To overcome the issue, it is replaced with an analogous alias added only for exposition (i.e., not as a public API) and defined in terms of C++20.

For comparison, this is the standard definition of std::projected_value_t:

template<indirectly_readable I, indirectly_regular_unary_invocable<I> Proj>
using projected_value_t = // freestanding
  remove_cvref_t<invoke_result_t<Proj&, iter_value_t<I>&>>;

It's a backward-compatible bug fix. Ideally it should be added to a revision of the specification v1.4.

[danhoeflinger]

Other than minor nitpick it LGTM