WG21-P3860R1 Make atomic_ref<T> Convertible To atomic_ref<const T>

This was officially voted in as a C++20 Defect Report during the November 2025 meeting.

No feature-test macro is being added, intentionally.

Followup to #5147, which we implemented unconditionally with #5213.

"I remember that we cleared this world out. We won. Is a future coming in which we will, eventually, truly, have won?"
- Ra, by qntm