Simplified `ui_stack_system` #48460

Job	Run time
ci	2m 24s
build (windows-latest)	5m 49s
check-bevy-internal-imports	5s
check-doc	11m 19s
build (ubuntu-latest)	2m 46s
build (macos-latest)	2m 20s
check-missing-examples-in-docs	30s
miri	9m 4s
run-examples-macos-metal	5m 19s
toml	1m 54s
typos	8s
check-missing-features-in-docs	33s
markdownlint	1m 9s
check-compiles	4m 38s
build-wasm	44s
build-wasm-atomics	1m 38s
msrv	1m 17s
	51m 37s

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