GPv3: UI: Rename buttons in layer group context menu

In a recent module meeting
https://devtalk.blender.org/t/2024-05-21-grease-pencil-module-meeting/34755
it was proposed to to make "Delete Group" delete everything
(the group with children) and use "Ungroup"
for just deleting the group while keeping the children.

This implements the suggested change that was agreed upon.

scripts/startup/bl_ui/properties_data_grease_pencil.py

@@ -200,8 +200,8 @@ class GREASE_PENCIL_MT_group_context_menu(Menu):
 
     def draw(self, context):
         layout = self.layout
-        layout.operator("grease_pencil.layer_group_remove", text="Delete Group").keep_children = True
-        layout.operator("grease_pencil.layer_group_remove", text="Delete Group and children").keep_children = False
+        layout.operator("grease_pencil.layer_group_remove", text="Delete Group").keep_children = False
+        layout.operator("grease_pencil.layer_group_remove", text="Ungroup").keep_children = True
 
 
 class DATA_PT_grease_pencil_layers(DataButtonsPanel, Panel):