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Bump third_party/ot-br-posix/repo from `bd15f71` to `5768fd5` #3509

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Run time

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Job	Run time
Chef - Linux CI Examples	6m 7s
Chef - ESP32 CI Examples	14m 6s
Chef - NRFConnect CI Examples	12m 5s
Chef - Telink CI Examples	11m 47s
	44m 5s