Simplified `ui_stack_system` #48456

Job	Run time
build (windows-latest)	6m 3s
check-bevy-internal-imports	4s
build (ubuntu-latest)	2m 24s
build (macos-latest)	1m 59s
check-doc	10m 48s
check-missing-examples-in-docs	31s
ci	2m 35s
miri	9m 47s
run-examples-macos-metal	4m 56s
toml	1m 52s
typos	6s
check-missing-features-in-docs	31s
markdownlint	1m 1s
check-compiles	4m 27s
build-wasm	53s
msrv	1m 20s
build-wasm-atomics	1m 34s
	50m 51s

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