Bump third_party/ot-br-posix/repo from `bd15f71` to `50ff1d1` #1577

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	1h 57m 14s
Build on Linux (gcc_debug)	1h 4m 4s
Build on Darwin (clang, python_lib, simulated)	2h 9m 6s
Build on Linux (python_lib)	29m 27s
Build on Linux (python lighting-app)	6m 10s
Build on Linux (coverage)	13m 55s
	5h 59m 56s

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