hud_elem_type deprecation fix breaks pre-5.9.0 Minetest

Since you recently switched from hud_elem_type to type in a1a489b6, this breaks all pre-5.9.0 clients because HUD elements become all garbled. Despite what the commit message claims, this change is NOT retrocompatible.

If you want to preserve-backwards-compability (and I hope you want to), you could assign the correct field name in a variable like so:

local hud_def_type_field
if minetest.features.hud_def_type_field then
   hud_def_type_field = "type"
else
   hud_def_type_field = "hud_elem_type"
end

-- ... lots of code ...

   player:hud_add({
      [hud_def_type_field] = "text",
      offset = { x = 0, y = 0 },
      -- etc.
   })

It's unfortunate this field was deprecated (in fact, I HATE it) but sadly, we have to live with that for now. ☹