Update Voxel demo for Godot 4.1.1

[Calinou]

• See [TRACKER] Porting the demos to Godot 4 #697.

Note: I've gotten a crash once after creating a flat world (couldn't reproduce it more than once) on 4.2.dev godotengine/godot@7831eedf1:

ERROR: Caller thread can't call this function in this node (/root). Use call_deferred() or call_thread_group() instead.
   at: propagate_notification (./scene/main/node.cpp:2218)

handle_crash: Program crashed with signal 11
Engine version: Godot Engine v4.2.dev.custom_build (7831eedf1c24805e6efdd5b7ee659c2095eaa765)
Dumping the backtrace. Please include this when reporting the bug to the project developer.
[1] /lib64/libc.so.6(+0x3dbb0) [0x7f622165fbb0] (??:0)
[2] godot() [0x83415f2] (/usr/include/c++/13/bits/atomic_base.h:505)
[3] godot() [0x83424e8] (/home/hugo/Documents/Git/godotengine/godot/./core/variant/dictionary.cpp:388)
[4] godot() [0x83452a0] (/home/hugo/Documents/Git/godotengine/godot/./core/variant/variant.cpp:1178 (discriminator 1))
[5] godot() [0x8352a0b] (/home/hugo/Documents/Git/godotengine/godot/./core/variant/variant.cpp:2922)
[6] godot() [0x87e1628] (/home/hugo/Documents/Git/godotengine/godot/./core/object/object.cpp:320)
[7] godot() [0x860668e] (/home/hugo/Documents/Git/godotengine/godot/./core/variant/variant_setget.cpp:286)
[8] godot() [0x2a7ad35] (/home/hugo/Documents/Git/godotengine/godot/./modules/gdscript/gdscript_vm.cpp:1142)
[9] godot() [0x290efbd] (/home/hugo/Documents/Git/godotengine/godot/./modules/gdscript/gdscript.cpp:1843 (discriminator 1))
[10] godot() [0x87e467f] (/home/hugo/Documents/Git/godotengine/godot/./core/object/object.cpp:744 (discriminator 1))
[11] godot() [0x8361954] (/home/hugo/Documents/Git/godotengine/godot/./core/variant/variant_call.cpp:1170 (discriminator 2))
[12] godot() [0x2a83cc3] (/home/hugo/Documents/Git/godotengine/godot/./modules/gdscript/gdscript_vm.cpp:1704)
[13] godot() [0x290efbd] (/home/hugo/Documents/Git/godotengine/godot/./modules/gdscript/gdscript.cpp:1843 (discriminator 1))
[14] godot() [0x87e467f] (/home/hugo/Documents/Git/godotengine/godot/./core/object/object.cpp:744 (discriminator 1))
[15] godot() [0x8361954] (/home/hugo/Documents/Git/godotengine/godot/./core/variant/variant_call.cpp:1170 (discriminator 2))
[16] godot() [0x2a83f78] (/home/hugo/Documents/Git/godotengine/godot/./modules/gdscript/gdscript_vm.cpp:1734)
[17] godot() [0x290efbd] (/home/hugo/Documents/Git/godotengine/godot/./modules/gdscript/gdscript.cpp:1843 (discriminator 1))
[18] godot() [0x87e467f] (/home/hugo/Documents/Git/godotengine/godot/./core/object/object.cpp:744 (discriminator 1))
[19] godot() [0x8339d40] (/home/hugo/Documents/Git/godotengine/godot/./core/variant/callable.cpp:62 (discriminator 1))
[20] godot() [0x7efedfb] (/home/hugo/Documents/Git/godotengine/godot/./core/core_bind.cpp:1236)
[21] godot() [0x7ffdc84] (/home/hugo/Documents/Git/godotengine/godot/./core/os/thread.cpp:63)
[22] godot() [0x8015cb1] (/usr/include/c++/13/bits/invoke.h:61)
[23] godot() [0x8015b7a] (/usr/include/c++/13/bits/invoke.h:97)
[24] godot() [0x8015a35] (/usr/include/c++/13/bits/std_thread.h:292)
[25] godot() [0x801332c] (/usr/include/c++/13/bits/std_thread.h:299)
[26] godot() [0x8011238] (/usr/include/c++/13/bits/std_thread.h:244)
[27] godot() [0x8a5a183] (msdf-error-correction.cpp:?)
[28] /lib64/libc.so.6(+0x8c947) [0x7f62216ae947] (??:0)
[29] /lib64/libc.so.6(+0x112870) [0x7f6221734870] (??:0)
-- END OF BACKTRACE --
================================================================
[1]    49615 IOT instruction (core dumped)  godot