Bump third_party/imgui/repo from `3c435c0` to `71c77c0` #5551

Job	Run time
Build on Linux (fake, gcc_release, clang, simulated)	1h 0m 48s
Build on Linux (gcc_debug)	1h 5m 23s
Build on Darwin (clang, simulated)	40m 13s
Build on Linux (python_lib)	24m 3s
Build on Linux (coverage)	14m 14s
Build on Linux (python lighting-app)	6m 49s
	3h 31m 30s

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