Bump third_party/openthread/repo from `aed9cd1` to `7f81453` #971

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	1h 58m 34s
Build on Darwin (clang, python_lib, simulated)	2h 30m 39s
Build on Linux (gcc_debug)	1h 1m 54s
Build on Linux (python_lib)	31m 1s
Build on Linux (python lighting-app)	10m 17s
Build on Linux (coverage)	14m 17s
	6h 26m 42s

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