Bump third_party/openthread/repo from `66eaeec` to `a8afffb` #1133

Job	Run time
Unit / Integration Tests (main)	12m 20s
Unit / Integration Tests (clang)	11m 15s
Unit / Integration Tests (mbedtls)	12m 35s
Unit / Integration Tests (rotating_device_id)	13m 13s
	49m 23s

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