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Bump third_party/openthread/repo from `b78b71b` to `ac7f751` #466

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Run time

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Job	Run time
Chef - ESP32 CI Examples	9m 8s
Chef - Linux CI Examples	6m 20s
Chef - NRFConnect CI Examples	8m 2s
Chef - Telink CI Examples	7m 40s
	31m 10s