submodule: bump openthread from `830572f` to `3a28b7b` #975

Job	Run time
cancel-previous-runs	8s
Test Suite network-forming	23m 36s
Test Suite commissioning	8m 47s
Test Suite connectivity	21m 50s
Test Suite network-latency	2m 13s
Test Suite multicast-performance	1m 57s
Test Suite otns-performance	2m 3s
Test Suite network-limits	1m 56s
	1h 2m 30s

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