Bump third_party/imgui/repo from `96839b4` to `6b7358e` #428

Job	Run time
Build on Darwin (clang, python_lib, simulated)	59m 43s
Build on Linux (fake, gcc_release, clang, simulated)	1h 26m 22s
Build on Linux (coverage)	9m 13s
Build on Linux (gcc_debug)	50m 18s
Build on Linux (python_lib)	20m 4s
Build on Linux (python lighting-app)	6m 7s
	3h 51m 47s

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