deprecate _format UDL in code using FMT_DEPRECATED

include/fmt/format.h

@@ -3056,7 +3056,7 @@ constexpr auto operator"" _a(const char* s, size_t) -> detail::udl_arg<char> {
     using namespace fmt::literals;
     std::string message = "The answer is {}"_format(42);
  */
-constexpr auto operator"" _format(const char* s, size_t n)
+FMT_DEPRECATED constexpr auto operator"" _format(const char* s, size_t n)
     -> detail::udl_formatter<char> {
   return {{s, n}};
 }