Add RenderSet::FinalCleanup for World::clear_entities (#14764)

# Objective

`World::clear_entities` is ambiguous with all of the other systems in
`RenderSet::Cleanup` because it access `&mut World`.

## Solution

I've added another system set variant, and made sure that this runs
after everything else.
 
## Testing

The `ambiguity_detection` example

## Migration Guide

`World::clear_entities` is now part of `RenderSet::PostCleanup` rather
than `RenderSet::Cleanup`. Your cleanup systems should likely stay in
`RenderSet::Cleanup`.

## Additional context

Spotted when working on #7386: this was responsible for a large number
of ambiguities.

This should be removed if / when #14449 is merged: there's no need to
call `clear_entities` at all if the rendering world is retained!

crates/bevy_render/src/lib.rs

@@ -104,7 +104,6 @@ pub struct RenderPlugin {
 
 /// The systems sets of the default [`App`] rendering schedule.
 ///
-/// that runs immediately after the matching system set.
 /// These can be useful for ordering, but you almost never want to add your systems to these sets.
 #[derive(Debug, Hash, PartialEq, Eq, Clone, SystemSet)]
 pub enum RenderSet {
@@ -138,6 +137,10 @@ pub enum RenderSet {
     Render,
     /// Cleanup render resources here.
     Cleanup,
+    /// Final cleanup occurs: all entities will be despawned.
+    ///
+    /// Runs after [`Cleanup`](RenderSet::Cleanup).
+    PostCleanup,
 }
 
 /// The main render schedule.
@@ -162,6 +165,7 @@ impl Render {
                 Prepare,
                 Render,
                 Cleanup,
+                PostCleanup,
             )
                 .chain(),
         );
@@ -469,7 +473,7 @@ unsafe fn initialize_render_app(app: &mut App) {
                     render_system,
                 )
                     .in_set(RenderSet::Render),
-                World::clear_entities.in_set(RenderSet::Cleanup),
+                World::clear_entities.in_set(RenderSet::PostCleanup),
             ),
         );