Bump third_party/ot-br-posix/repo from `bd15f71` to `7fd5bd1` #5567

Job	Run time
Build on Darwin (clang, simulated)	46m 25s
Build on Linux (fake, gcc_release, clang, simulated)	54m 2s
Build on Linux (coverage)	14m 16s
Build on Linux (python_lib)	23m 58s
Build on Linux (gcc_debug)	1h 4m 53s
Build on Linux (python lighting-app)	7m 13s
	3h 30m 47s

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