Bump third_party/ot-br-posix/repo from `bd15f71` to `b574b6c` #5465

Job	Run time
Unit / Integration Tests (main)	16m 6s
Unit / Integration Tests (clang)	17m 22s
Unit / Integration Tests (mbedtls)	15m 31s
Unit / Integration Tests (rotating_device_id)	15m 33s
Unit / Integration Tests (icd)	45m 33s
	1h 50m 5s

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