Bump third_party/openthread/repo from `b78b71b` to `ac7f751` #1171

Job	Run time
Unit / Integration Tests (main)	12m 45s
Unit / Integration Tests (clang)	12m 51s
Unit / Integration Tests (mbedtls)	12m 50s
Unit / Integration Tests (rotating_device_id)	13m 8s
Unit / Integration Tests (icd)	22m 57s
	1h 14m 31s

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