Bump third_party/openthread/repo from `786bd7f` to `1d81283` #902

Job	Run time
REPL Tests - Linux	37m 13s
Test Suites - Darwin (no-ble-asan-clang)	1h 3m 54s
Test Suites - Linux (no-ble-tsan-clang)	1h 30m 43s
Test Suites - Darwin (no-ble-tsan-clang)	1h 45m 33s
REPL Tests - Darwin	0s
	4h 57m 23s

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