Bump third_party/imgui/repo from `2860d7b` to `df31774` #1480

Job	Run time
REPL Tests - Darwin	0s
Test Suites - Darwin (no-ble-no-shell-asan-clang)	1h 19m 22s
REPL Tests - Linux	1h 16m 5s
Test Suites - Darwin (no-ble-no-shell-tsan-clang)	1h 56m 12s
Test Suites - Linux (no-ble-no-shell-tsan-clang)	1h 47m 8s
	6h 18m 47s

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