Bump third_party/openthread/repo from `287dbfa` to `147de7e` #1051

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	51m 51s
Build on Linux (gcc_debug)	1h 2m 4s
Build on Linux (python_lib)	30m 57s
Build on Linux (python lighting-app)	7m 3s
Build on Darwin (clang, simulated)	51m 52s
Build on Linux (coverage)	12m 43s
	3h 36m 30s

Provide feedback