Bump third_party/imgui/repo from `5288687` to `27e83c2` #1202

Job	Run time
REPL Tests - Linux	35m 26s
Test Suites - Darwin (no-ble-asan-clang)	1h 29m 8s
Test Suites - Linux (no-ble-tsan-clang)	1h 28m 39s
Test Suites - Darwin (no-ble-tsan-clang)	1h 44m 55s
REPL Tests - Darwin	0s
	5h 18m 8s

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