Bump third_party/openthread/repo from `102a631` to `851e00f` #857

Job	Run time
Unit / Integration Tests (main)	18m 54s
Unit / Integration Tests (clang)	15m 24s
Unit / Integration Tests (mbedtls)	20m 3s
Unit / Integration Tests (rotating_device_id)	19m 15s
	1h 13m 36s

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