Bump third_party/ot-br-posix/repo from `bd15f71` to `5c66507` #1616

Job	Run time
Unit / Integration Tests (main)	17m 56s
Unit / Integration Tests (clang)	18m 11s
Unit / Integration Tests (mbedtls)	16m 59s
Unit / Integration Tests (rotating_device_id)	16m 20s
Unit / Integration Tests (icd)	45m 40s
	1h 55m 6s

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