Upgrade to `objc2 0.6` and `objc2-metal 0.3` #1493

Job	Run time
Check MSRV (1.71.0) (ubuntu-latest, vulkan)	27s
Build documentation (windows-latest)	2m 7s
Check MSRV (1.71.0) (windows-latest, vulkan,d3d12)	1m 7s
Clippy (ubuntu-latest, vulkan,visualizer)	23s
Build documentation (macos-latest)	23s
Check MSRV (1.71.0) (macos-latest, vulkan,metal)	37s
Clippy (windows-latest, vulkan,visualizer,d3d12,public-winapi)	2m 13s
Clippy (macos-latest, vulkan,visualizer,metal)	20s
Rustfmt	3s
Test Suite (ubuntu-latest, vulkan,visualizer)	25s
readme	4s
Test Suite (windows-latest, vulkan,visualizer,d3d12,public-winapi)	2m 21s
Test Suite (macos-latest, vulkan,visualizer,metal)	42s
	11m 12s

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