a11y: Replace "enable animations" with "reduced motion"
The "enable animations" setting is not a good accessibility toggle: it's meant to be used as a performance toggle (e.g. for applications running inside a VM or on a remote desktop). This is why #3099 (closed) required adding a whole disclaimer about it.
The reduced-motion accessibility setting is going to be proxied by the GNOME desktop portal to applications and toolkits.