submodule: bump openthread from `830572f` to `a759a4a` #968

Job	Run time
cancel-previous-runs	8s
Test Suite network-forming	23m 37s
Test Suite commissioning	9m 3s
Test Suite connectivity	23m 2s
Test Suite network-latency	2m 12s
Test Suite multicast-performance	1m 59s
Test Suite otns-performance	2m 6s
Test Suite network-limits	2m 0s
	1h 4m 7s

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