Bump third_party/ot-br-posix/repo from `9bdaa91` to `bd15f71` #268

Job	Run time
Unit / Integration Tests (main)	14m 32s
Unit / Integration Tests (clang)	13m 34s
Unit / Integration Tests (mbedtls)	13m 52s
Unit / Integration Tests (rotating_device_id)	15m 33s
Unit / Integration Tests (icd)	43m 47s
	1h 41m 18s

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