Bump third_party/imgui/repo from `03298fe` to `240ab58` #1226

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 11m 14s
Build on Linux (fake, gcc_release, clang, simulated)	1h 23m 30s
Build on Linux (coverage)	1h 28m 56s
Build on Linux (gcc_debug)	47m 4s
Build on Linux (python_lib)	19m 6s
Build on Linux (python lighting-app)	6m 5s
	5h 15m 55s

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