Bump third_party/openthread/repo from `f0b6fce` to `518a849` #271

Job	Run time
Test Suites - Linux (no-ble-tsan-clang)	1h 45m 44s
Test Suites - Darwin (no-ble-asan-clang)	1h 15m 48s
Test Suites - Darwin (no-ble-tsan-clang)	2h 7m 6s
REPL Tests - Darwin	0s
REPL Tests - Linux	49m 25s
	5h 58m 3s

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