Bump third_party/imgui/repo from `3c435c0` to `aa23f38` #5609

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	54m 36s
Build on Linux (gcc_debug)	58m 41s
Build on Linux (python_lib)	22m 10s
Build on Linux (python lighting-app)	5m 53s
Build on Darwin (clang, simulated)	54m 3s
Build on Linux (coverage)	14m 26s
	3h 29m 49s

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