Bump third_party/imgui/repo from `3c435c0` to `2860d7b` #98865

Job	Run time
REPL Tests - Linux	1h 53m 42s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 13m 33s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	1h 44m 51s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	1h 43m 56s
REPL Tests - Darwin	0s
	6h 36m 2s

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