Changing at-rest z-index for Reorder.Item to "unset"

[mattgperry]

Fixes #1313

[codesandbox-ci]

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Sandbox	Source
Framer Motion: Simple animation	Configuration
App Store UI using React and Framer Motion	Configuration
Framer Motion: Reorder animation	Configuration
Framer Motion: growing item positionTransition issue	Configuration
Framer Motion: Image lightbox	Configuration
Framer Motion 5: Drag to reorder lists (forked)	Issue #1313

[dfelber]

FYI: This introduced a rendering bug (Dragged item not always on top) when reordering items.
You can see it even on the official page: https://www.framer.com/motion/reorder/ in the first example. (uses 6.2.8)
https://codesandbox.io/p/sandbox/framer-motion-5-drag-to-reorder-lists-uonye

In the example using drag triggers it behaves correctly (uses 5.0.0-rc.4)
https://codesandbox.io/p/sandbox/framer-motion-5-drag-to-reorder-lists-with-drag-handle-j9enw