Bump third_party/openthread/repo from `2aeb8b8` to `5bd9de5` #1603

Job	Run time
Build on Darwin (clang, python_lib, simulated)	2h 40m 15s
Build on Linux (fake, gcc_release, clang, simulated)	1h 58m 2s
Build on Linux (coverage)	14m 10s
Build on Linux (gcc_debug)	1h 1m 56s
Build on Linux (python_lib)	22m 39s
Build on Linux (python lighting-app)	6m 55s
	6h 23m 57s

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