Bump third_party/ot-br-posix/repo from `bd15f71` to `277c9e0` #1666

Job	Run time
Unit / Integration Tests (main)	16m 6s
Unit / Integration Tests (clang)	21m 3s
Unit / Integration Tests (mbedtls)	16m 13s
Unit / Integration Tests (rotating_device_id)	15m 39s
Unit / Integration Tests (icd)	45m 58s
	1h 54m 59s

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