submodule: bump openthread from `830572f` to `a1816c1` #959

Job	Run time
Test Suite network-forming	22m 47s
cancel-previous-runs	8s
Test Suite commissioning	10m 9s
Test Suite connectivity	13m 6s
Test Suite network-latency	1m 56s
Test Suite multicast-performance	1m 54s
Test Suite otns-performance	1m 55s
Test Suite network-limits	1m 46s
	53m 41s

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