Bump third_party/openthread/repo from `bf587dd` to `08a7600` #429

Job	Run time
Build on Linux (gcc_debug)	48m 36s
Build on Darwin (clang, python_lib, simulated)	1h 10m 33s
Build on Linux (fake, gcc_release, clang, simulated)	1h 28m 8s
Build on Linux (coverage)	11m 17s
Build on Linux (python_lib)	19m 34s
Build on Linux (python lighting-app)	6m 3s
	4h 4m 11s

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