World creation screen background can no longer be retextured individually from other menu backgrounds

Discovered while testing MC-262503.

The bug

As of 24w09a, it is no longer possible to retexture the background used for the world creation screen independently of the background used for other non-list menus.

How to reproduce

Two resource packs are attached to this issue - one for 24w07a and one for 24w10a - that replace the relevant texture files.

How to fix

Add back a dedicated texture file for the world creation menu background. In the default resource pack, this would be a direct copy of the existing menu_background.png, and in Programmer Art and High Contrast, it'd be a copy of their versions of menu_background.png