Simplified `ui_stack_system` #48461

Job	Run time
build (windows-latest)	2m 5s
check-bevy-internal-imports	6s
build (ubuntu-latest)	2m 20s
check-doc	2m 20s
build (macos-latest)	1m 56s
check-missing-examples-in-docs	40s
miri	2m 21s
ci	2m 17s
toml	2m 10s
run-examples-macos-metal	2m 21s
typos	8s
check-missing-features-in-docs	29s
markdownlint	1m 9s
check-compiles	1s
msrv	1s
build-wasm-atomics	1s
build-wasm	1s
	20m 26s

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