Bump third_party/openthread/repo from `287dbfa` to `473af53` #435

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	54m 9s
Build on Linux (gcc_debug)	1h 4m 2s
Build on Linux (python_lib)	23m 45s
Build on Darwin (clang, simulated)	51m 5s
Build on Linux (coverage)	13m 36s
Build on Linux (python lighting-app)	12m 4s
	3h 38m 41s

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