Bump third_party/openthread/repo from `2aeb8b8` to `7b72e97` #5575

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	52m 12s
Build on Linux (gcc_debug)	1h 0m 36s
Build on Darwin (clang, simulated)	49m 34s
Build on Linux (coverage)	15m 3s
Build on Linux (python_lib)	21m 4s
Build on Linux (python lighting-app)	5m 25s
	3h 23m 54s

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