Add RenderSet::FinalCleanup for World::clear_entities

[alice-i-cecile]

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!

[JMS55]

Kinda gross, but retained rendering world will fix it (and give very nice perf improvements!)

[ItsDoot]

Nit: PostCleanup matches our schedule naming scheme better

[ItsDoot]

Don't forget to update the migration guide 👍

[JMS55]

I actually dislike PostCleanup more, as it implies that all cleanup is actually done and that it's not doing cleanup itself... But this whole thing is sucky, and hopefully fixed soon. We really need to get retained render world merged asap imo.