Bump third_party/openthread/repo from `66eaeec` to `a8afffb` #1132

Job	Run time
Unit / Integration Tests (main)	12m 41s
Unit / Integration Tests (clang)	11m 48s
Unit / Integration Tests (mbedtls)	12m 3s
Unit / Integration Tests (rotating_device_id)	12m 24s
	48m 56s

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