Bump third_party/ot-br-posix/repo from `bd15f71` to `b9210e7` #296

Job	Run time
Unit / Integration Tests (main)	17m 45s
Unit / Integration Tests (clang)	16m 52s
Unit / Integration Tests (mbedtls)	16m 27s
Unit / Integration Tests (rotating_device_id)	17m 9s
Unit / Integration Tests (icd)	46m 36s
	1h 54m 49s

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