Bump third_party/imgui/repo from `3c435c0` to `28caa22` #1207

Job	Run time
REPL Tests - Darwin	0s
REPL Tests - Linux	1h 5m 26s
Test Suites - Darwin (no-ble-asan-clang)	1h 21m 50s
Test Suites - Darwin (no-ble-tsan-clang)	1h 47m 21s
Test Suites - Linux (no-ble-tsan-clang)	1h 46m 44s
	6h 1m 21s

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