Bump third_party/openthread/repo from `287dbfa` to `b0984af` #2394

Job	Run time
Build on Darwin (clang, simulated)	46m 13s
Build on Linux (fake, gcc_release, clang, simulated)	54m 53s
Build on Linux (coverage)	13m 7s
Build on Linux (gcc_debug)	1h 6m 27s
Build on Linux (python_lib)	25m 18s
Build on Linux (python lighting-app)	8m 3s
	3h 34m 1s

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