Bump third_party/openthread/repo from `2aeb8b8` to `b42be4c` #312

Job	Run time
Unit / Integration Tests (main)	15m 5s
Unit / Integration Tests (clang)	17m 33s
Unit / Integration Tests (mbedtls)	17m 18s
Unit / Integration Tests (rotating_device_id)	15m 9s
Unit / Integration Tests (icd)	45m 4s
	1h 50m 9s

Provide feedback