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Bump third_party/ot-br-posix/repo from `bd15f71` to `f2a7208` #342

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Run time

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Job	Run time
Chef - ESP32 CI Examples	8m 40s
Chef - Linux CI Examples	5m 23s
Chef - NRFConnect CI Examples	6m 36s
Chef - Telink CI Examples	12m 8s
	32m 47s