EEVEE: Remove all remaining reference of EEVEE next

This changes the engine identifier back to `BLENDER_EEVEE`.

We keep the `BLENDER_EEVEE_NEXT` identifier around for
versioning reasons (have to detect when it is the active
engine of a older file).

This also rename a bunch of pannels that were using `next`
in their name.

This is a breaking change for Addons compatibility.

Pull Request: https://projects.blender.org/blender/blender/pulls/140282

tools/utils_doc/code_layout_diagram.py

@@ -1094,7 +1094,7 @@ def main():
     VFONT_FROM_STYLE["mono"] = bpy.data.fonts.load(FONT_FILE_MONO)
 
     scene = bpy.context.scene
-    scene.render.engine = 'BLENDER_EEVEE_NEXT'
+    scene.render.engine = 'BLENDER_EEVEE'
 
     # Without this, the whites are gray.
     scene.view_settings.view_transform = "Standard"