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Bump third_party/ot-br-posix/repo from `bd15f71` to `b9210e7` #509

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Run time

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Job	Run time
Chef - ESP32 CI Examples	13m 29s
Chef - Linux CI Examples	5m 54s
Chef - NRFConnect CI Examples	6m 46s
Chef - Telink CI Examples	7m 9s
	33m 18s