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Bump third_party/ot-br-posix/repo from `bd15f71` to `6934ff0` #420

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Run time

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Job	Run time
Chef - ESP32 CI Examples	8m 47s
Chef - Linux CI Examples	6m 15s
Chef - NRFConnect CI Examples	7m 17s
Chef - Telink CI Examples	6m 59s
	29m 18s