Bump third_party/imgui/repo from `3c435c0` to `2860d7b` #114924

Job	Run time
Build on Linux (gcc_debug)	50m 43s
Build on Darwin (clang, simulated)	38m 48s
Build on Linux (fake, gcc_release, clang, simulated)	47m 44s
Build on Linux (coverage)	18m 35s
Build on Linux (python_lib)	19m 22s
Build on Linux (python lighting-app)	4m 44s
	2h 59m 56s

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