`doors` mod uses field `sound` as string (crash in 5.6+)

[BuckarooBanzay]

Minetest 5.7 crashes for me:

2023-03-10 10:40:29: ERROR[Main]: ModError: Failed to load and run script from /home/thomas/.minetest/games/RealMoon2021/mods/MTG_MODS/doors/init.lua:
2023-03-10 10:40:29: ERROR[Main]: /usr/local/share/minetest/builtin/game/register.lua:199: bad argument #5 to 'register_item_raw' (table expected, got string)
2023-03-10 10:40:29: ERROR[Main]: stack traceback:
2023-03-10 10:40:29: ERROR[Main]: 	[C]: in function 'register_item_raw'
2023-03-10 10:40:29: ERROR[Main]: 	/usr/local/share/minetest/builtin/game/register.lua:199: in function 'register_item'
2023-03-10 10:40:29: ERROR[Main]: 	/usr/local/share/minetest/builtin/game/register.lua:225: in function 'register_node'
2023-03-10 10:40:29: ERROR[Main]: 	...minetest/games/RealMoon2021/mods/MTG_MODS/doors/init.lua:792: in function 'register_fencegate'
2023-03-10 10:40:29: ERROR[Main]: 	...minetest/games/RealMoon2021/mods/MTG_MODS/doors/init.lua:804: in main chunk

the reason is that the sound field in the to-be registered door nodes was set with a string (should be a table i guess):

Here:

RealMoon2021/mods/MTG_MODS/doors/init.lua

Line 775 in 34fdbe6

fence_closed.sound = "doors_fencegate_open"

and here:

RealMoon2021/mods/MTG_MODS/doors/init.lua

Line 784 in 34fdbe6

fence_open.sound = "doors_fencegate_close"

Removing those lines fixes the crash