fix universal: fix ForwardLike in C++23 to behave same as std::forwad_like

[emikg777]

utils::ForwardLike used return x; for the non-const lvalue-like case.

This no longer matches std::forward_like in C++23. Since P2266R3 ("Simpler implicit move"), the C++23 return statement rules treat a move-eligible returned expression as an xvalue. In standard wording this is the return statement wording in [stmt.return]: if the returned expression is move-eligible, it is treated as an xvalue. std::forward_like<T> still has to preserve the reference category requested by T&&, so ForwardLike<int&>(...) must produce an lvalue reference.

The existing forward_like_test.cpp fails with gcc 15.2.0:

/home/egoncharov/bcore/cmake-build-debug/_deps/userver-src/universal/src/utils/forward_like_test.cpp:55:27:   required from here
   55 |     TestsTwoForwards<int&>(char{});
      |     ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
/home/egoncharov/bcore/cmake-build-debug/_deps/userver-src/universal/src/utils/forward_like_test.cpp:36:24: error: static assertion failed
   36 |     static_assert(std::is_same_v<
      |                   ~~~~~^~~~~~~~~~
   37 |                   decltype(std::forward_like<T>(std::forward<U>(x))),
      |                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   38 |                   decltype(utils::ForwardLike<T>(std::forward<U>(x)))>);
      |                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/egoncharov/bcore/cmake-build-debug/_deps/userver-src/universal/src/utils/forward_like_test.cpp:36:24: note: 'std::is_same_v<char&, char&&>' evaluates to false

This change implements ForwardLike via explicit casts to the expected reference type instead of relying on return x, std::move, or std::as_const.

Here is minimal demonsration of difference between original ForwardLike and std::forward_like in C++23:
https://godbolt.org/z/s7WdxYaWe

The implementation now:

• propagates const from T to the returned reference type;
• uses T&& to decide whether the result should be an lvalue reference or an rvalue reference;
• avoids dependence on C++23 return statement implicit move rules;
• matches std::forward_like semantics.

Testing:

• Built a userver-based project with CMAKE_CXX_STANDARD=23.
• Built and ran userver-universal-unittest.
• Full userver-universal-unittest run: 1671/1672 tests passed. The only failure was LogFilepath.UserverCroppedCorrectly, which appears to be path-layout-sensitive when userver is built as a CPM dependency under _deps/userver-src.