Bump third_party/openthread/repo from `287dbfa` to `b0984af` #2382

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	6h 0m 3s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 52m 27s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	1h 57m 15s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	1h 58m 14s
	11h 47m 59s

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