Bump third_party/openthread/repo from `2aeb8b8` to `7886083` #406

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 40m 50s
Build on Linux (coverage)	13m 49s
Build on Linux (fake, gcc_release, clang, simulated)	1h 59m 48s
Build on Linux (gcc_debug)	1h 3m 26s
Build on Linux (python_lib)	23m 13s
Build on Linux (python lighting-app)	5m 41s
	5h 26m 47s

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