Bump third_party/openthread/repo from `2aeb8b8` to `3a28b7b` #2352

Job	Run time
Unit / Integration Tests (main)	15m 33s
Unit / Integration Tests (clang)	18m 2s
Unit / Integration Tests (mbedtls)	17m 1s
Unit / Integration Tests (rotating_device_id)	16m 28s
Unit / Integration Tests (icd)	44m 57s
	1h 52m 1s

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