Bump third_party/openthread/repo from `f0b6fce` to `81542ca` #2401

Job	Run time
Build on Darwin (clang, python_lib, simulated)	2h 7m 3s
Build on Linux (coverage)	16m 22s
Build on Linux (fake, gcc_release, clang, simulated)	2h 12m 0s
Build on Linux (python_lib)	31m 5s
Build on Linux (gcc_debug)	1h 8m 57s
Build on Linux (python lighting-app)	10m 13s
	6h 25m 40s

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