Bump third_party/openthread/repo from `2aeb8b8` to `971f05f` #5581

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	52m 52s
Build on Linux (gcc_debug)	59m 17s
Build on Linux (python_lib)	22m 25s
Build on Linux (python lighting-app)	5m 32s
Build on Darwin (clang, simulated)	42m 34s
Build on Linux (coverage)	13m 55s
	3h 16m 35s

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