Server-side death formspec relies on 5.17.0-only feature

copied from https://github.com/luanti-org/luanti/pull/17365#discussion_r3993917689:

we forgot something actually: since this is server-side code and halign only exists in 5.17.0, the death screen will now effectively look incorrect for all clients 5.16.x and older.

so this function will need a check for the player's supported formspec_version