Internal root canvas group does not start with underscore

[winston-yallow]

Godot version

4.0.dev (daa055d)

System information

any system

Issue description

The root_canvas* group is not starting with an underscore. According to the docs all internal groups should start with _

I have no idea what the group is used for, but it seems to be defined here: https://github.com/godotengine/godot/blob/daa055d47/scene/main/canvas_item.cpp#L233

Steps to reproduce

Get the groups of a canvas item that is currently the canvas root.

Minimal reproduction project

No response

[winston-yallow]

This seems to exist in 3.x too, I just never noticed it there:
https://github.com/godotengine/godot/blob/3.x/scene/2d/canvas_item.cpp#L533