Bump third_party/openthread/repo from `287dbfa` to `fb3b265` #2370

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 33m 48s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 25m 24s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	1h 57m 45s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	1h 43m 10s
	6h 40m 7s

Provide feedback