Bump third_party/openthread/repo from `66eaeec` to `f239c49` #1108

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 7m 29s
Build on Linux (fake, gcc_release, clang, simulated)	2h 11m 33s
Build on Linux (coverage)	9m 51s
Build on Linux (gcc_debug)	35m 48s
Build on Linux (python_lib)	17m 6s
Build on Linux (python lighting-app)	6m 13s
	4h 28m 0s

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