Refactor: Unify vertex and sculpt colors into new

color attribute system.

This commit removes sculpt colors from experimental
status and unifies it with vertex colors. It
introduces the concept of "color attributes", which
are any attributes that represents colors.  Color
attributes can be represented with byte or floating-point
numbers and can be stored in either vertices or
face corners.

Color attributes share a common namespace
(so you can no longer have a floating-point
sculpt color attribute and a byte vertex color
attribute with the same name).

Note: this commit does not include vertex paint mode,
      which is a separate patch, see:
      https://developer.blender.org/D14179

Differential Revision: https://developer.blender.org/D12587
Ref D12587

release/scripts/startup/bl_ui/properties_material_gpencil.py

@@ -30,8 +30,8 @@ class GPENCIL_MT_material_context_menu(Menu):
 
         layout.separator()
 
-        layout.operator("gpencil.material_to_vertex_color", text="Convert Materials to Vertex Color")
-        layout.operator("gpencil.extract_palette_vertex", text="Extract Palette from Vertex Color")
+        layout.operator("gpencil.material_to_vertex_color", text="Convert Materials to Color Attribute")
+        layout.operator("gpencil.extract_palette_vertex", text="Extract Palette from Color Attribute")
 
         layout.separator()