Bump third_party/openthread/repo from `f0b6fce` to `6e46e2e` #583

Job	Run time
Unit / Integration Tests (main)	18m 49s
Unit / Integration Tests (clang)	17m 58s
Unit / Integration Tests (mbedtls)	17m 11s
Unit / Integration Tests (rotating_device_id)	19m 56s
Unit / Integration Tests (icd)	48m 41s
	2h 2m 35s

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