Deprecate `LoadAndSave` asset processor

[alice-i-cecile]

Technically LoadAndSave also works. All of cart's early asset processors worked in the AssetSaver, instead of a separate transform step. We might want to stop doing that and deprecate LoadAndSave.

Originally posted by @JMS55 in #15058 (comment)

[alice-i-cecile]

I agree with this: we should have one clear way to do this, and the three-step process is much clearer and more flexible.