Bump third_party/openthread/repo from `786bd7f` to `1d81283` #902

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 31m 24s
Build on Linux (fake, gcc_release, clang, simulated)	1h 50m 26s
Build on Linux (coverage)	13m 50s
Build on Linux (gcc_debug)	53m 0s
Build on Linux (python_lib)	30m 1s
	4h 58m 41s

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