Bump third_party/openthread/repo from `786bd7f` to `1d81283` #902

Job	Run time
Unit / Integration Tests (main)	21m 32s
Unit / Integration Tests (clang)	15m 10s
Unit / Integration Tests (mbedtls)	19m 14s
Unit / Integration Tests (rotating_device_id)	20m 18s
	1h 16m 14s

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