Update qxp::function_ref to P0792R14

Currently function_ref is at R9 state, but R14 is out (R10 adds std::nontype support to extend lifetime of function pointers).

Since reporting this, R14 has been merged for C++26, so qxp::function_ref should now get the fall-back to std::function_ref and be renamed q26::function_ref. However: we already use it in private and QTestLib public ABI, so switching to the fall-back will break BC between C++23 and C++26 builds (incl. the case where Qt is built in one of the two and the user project in the other).

As this thing is immensely useful, maybe make qxp::function_ref public API as Qt::function_ref, to be replaced by std::function_ref come Qt 7, and keep q26::function_ref as a compile-time switch for non-ABI use?

Correction: R14 is not merged, yet.