Bump third_party/openthread/repo from `2aeb8b8` to `7886083` #5548

Job	Run time
Build on Darwin (clang, simulated)	53m 44s
Build on Linux (fake, gcc_release, clang, simulated)	52m 23s
Build on Linux (coverage)	14m 15s
Build on Linux (gcc_debug)	1h 2m 54s
Build on Linux (python_lib)	23m 18s
Build on Linux (python lighting-app)	14m 49s
	3h 41m 23s

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