Bump third_party/imgui/repo from `5288687` to `27e83c2` #1201

Job	Run time
REPL Tests - Linux	32m 59s
Test Suites - Darwin (no-ble-asan-clang)	1h 46m 0s
Test Suites - Linux (no-ble-tsan-clang)	1h 42m 14s
Test Suites - Darwin (no-ble-tsan-clang)	2h 16m 0s
REPL Tests - Darwin	0s
	6h 17m 13s

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