0.12 Migration: No mention of batch phase being removed

[NiseVoid]

While looking into migrating kayak_ui I ran into batch_phase_system/BatchedPhaseItem being gone, but the migration doesn't seem to give any info on how to migrate things that depended on it. Presumably related to batching changes or the render schedule changes

[superdump]

@StarArawn pointed this out on Discord when working on porting kayak. Kayak is closer to the sprite rendering pipeline and that currently uses special-purpose code for batching so they said they would base the port on bevy’s new sprite rendering approach.

That said, migration information is needed for the render set reorder and batching. I’m happy to help anyone going through that process. Poke me on discord and we’ll make sure the information gets into the migration guide.