Bump third_party/openthread/repo from `2aeb8b8` to `f6cee79` #5606

Job	Run time
Build on Darwin (clang, simulated)	47m 54s
Build on Linux (fake, gcc_release, clang, simulated)	51m 45s
Build on Linux (gcc_debug)	1h 6m 25s
Build on Linux (python_lib)	22m 59s
Build on Linux (python lighting-app)	5m 33s
Build on Linux (coverage)	13m 47s
	3h 28m 23s

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