Bump third_party/imgui/repo from `3c435c0` to `923ca47` #2411

Job	Run time
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 30m 1s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	1h 44m 32s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	2h 10m 27s
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 36m 39s
	7h 1m 39s

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