Bump third_party/imgui/repo from `3c435c0` to `2860d7b` #110791

Job	Run time
Unit / Integration Tests (main)	14m 11s
Unit / Integration Tests (clang)	18m 52s
Unit / Integration Tests (mbedtls)	13m 38s
Unit / Integration Tests (rotating_device_id)	13m 30s
Unit / Integration Tests (icd)	47m 52s
	1h 48m 3s

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