The value format of `_spawnable_scenes` property of `MultiplayerSpawner` in .tscn is different between 4.4.dev3 and 4.4.dev6

### Tested versions

- Reproducible in 4.4.dev6 official and 4.4.dev3 official

### System information

Godot v4.4.dev3 - Windows 10.0.22631 - Multi-window, 1 monitor - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3060 Ti (NVIDIA; 32.0.15.6094) - 13th Gen Intel(R) Core(TM) i5-13600KF (20 threads)

### Issue description

When use 4.4.dev3 and set the property in Node `MultiplayerSpawner`'s `Auto Spawn List`, the saved value in .tscn is like:
```
[node name="Slot1Spawner" type="MultiplayerSpawner" parent="."]
_spawnable_scenes = PackedStringArray("res://player_head.tscn")
```

When use 4.4.dev6 and do the same, the saved value in .tscn is like:
```
[node name="Slot1Spawner" type="MultiplayerSpawner" parent="."]
_spawnable_scenes = PackedStringArray("uid://docb3aisq4txw")
```

Is this supposed to happen? A bug or not? I think using PackedStringArray("res://player_head.tscn") is better than using PackedStringArray("uid://docb3aisq4txw") but I'm not so sure.

Can an older version Godot understand `_spawnable_scenes = PackedStringArray("uid://docb3aisq4txw")` ? This behavior might **breaks compatibility** which is not mentioned before.


### Steps to reproduce

1. using 4.4.dev3, create a project
2. create a scene
3. add a `MultiplayerSpawner` node
4. edit it's property `Auto Spawn List` then add an element, then choose another scene.
5. save and see the value saved in .tscn file
6. Do the same in 4.4.dev6 and compare .tscn file.

### Minimal reproduction project (MRP)

NA

Start with the listed reproduction steps in Godot 4.4.dev3 and 4.4.dev6, then compare the saved .tscn values for MultiplayerSpawner's Auto Spawn List. Check whether older Godot versions can read the uid:// form and whether the change is intended. Done means the compatibility behavior is confirmed and the issue is either documented or corrected.

Written by the indexing model from the issue text.