Bump third_party/openthread/repo from `aed9cd1` to `7f81453` #971

Job	Run time
Unit / Integration Tests (main)	16m 30s
Unit / Integration Tests (clang)	16m 57s
Unit / Integration Tests (mbedtls)	16m 41s
Unit / Integration Tests (rotating_device_id)	16m 31s
Unit / Integration Tests (icd)	45m 18s
	1h 51m 57s

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