Bump third_party/openthread/repo from `287dbfa` to `fb3b265` #2382

Job	Run time
Unit / Integration Tests (main)	16m 26s
Unit / Integration Tests (clang)	18m 29s
Unit / Integration Tests (mbedtls)	17m 9s
Unit / Integration Tests (rotating_device_id)	18m 0s
Unit / Integration Tests (icd)	49m 51s
	1h 59m 55s

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