Bump third_party/openthread/repo from `2aeb8b8` to `b8d2e5d` #394

Job	Run time
Build on Darwin (clang, python_lib, simulated)	2h 16m 41s
Build on Linux (fake, gcc_release, clang, simulated)	1h 58m 25s
Build on Linux (coverage)	15m 3s
Build on Linux (gcc_debug)	59m 3s
Build on Linux (python_lib)	23m 6s
Build on Linux (python lighting-app)	6m 41s
	5h 58m 59s

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