Bump third_party/openthread/repo from `287dbfa` to `ada03e6` #429

Job	Run time
Build on Darwin (clang, simulated)	1h 1m 11s
Build on Linux (fake, gcc_release, clang, simulated)	1h 3m 17s
Build on Linux (coverage)	19m 44s
Build on Linux (gcc_debug)	1h 8m 24s
Build on Linux (python_lib)	31m 19s
Build on Linux (python lighting-app)	12m 17s
	4h 16m 12s

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