submodule: bump openthread from `830572f` to `666a9bd` #960

Job	Run time
cancel-previous-runs	5s
Test Suite network-forming	22m 45s
Test Suite commissioning	10m 15s
Test Suite connectivity	23m 21s
Test Suite network-latency	2m 4s
Test Suite multicast-performance	2m 12s
Test Suite otns-performance	1m 46s
Test Suite network-limits	2m 3s
	1h 4m 31s

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