Bump third_party/imgui/repo from `3c435c0` to `aa23f38` #5466

Job	Run time
Unit / Integration Tests (main)	15m 42s
Unit / Integration Tests (clang)	17m 46s
Unit / Integration Tests (mbedtls)	15m 59s
Unit / Integration Tests (rotating_device_id)	16m 26s
Unit / Integration Tests (icd)	49m 23s
	1h 55m 16s

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