Hide `fixed_update` behind a feature flag #26695

Job	Run time
build-and-install-on-iOS	0s
build-android	0s
run-examples-linux-vulkan	0s
build-without-default-features	0s
check-example-showcase-patches-still-work	0s
check-unused-dependencies	0s
run-examples-on-wasm	0s
run-examples-on-windows-dx12	0s
build-without-default-features-status	0s
	0s

Provide feedback