k_priority_queue: Fix concepts use

- For `std::same_as`, add missing include of `<concepts>`.

- For `std::convertible_to`, create a replacement in `common/concepts.h`
  and use that instead.

  This would also be found in `<concepts>`, but unlike `std::same_as`,
  `std::convertible_to` is not yet implemented in libc++, LLVM's STL
  implementation - not even in master.  (In fact, `std::same_as` is the
  *only* concept currently implemented.  For some reason.)

src/common/concepts.h

@@ -31,4 +31,8 @@ concept DerivedFrom = requires {
     std::is_convertible_v<const volatile Derived*, const volatile Base*>;
 };
 
+// TODO: Replace with std::convertible_to when libc++ implements it.
+template <typename From, typename To>
+concept ConvertibleTo = std::is_convertible_v<From, To>;
+
 } // namespace Common