I assume you're using the React SDK. That's on
version 1.4.1 now; 0.21.0 is over a year old, hence the warning. You can click on the list item on the left for more info.
Regarding 0.21.0 vs 0.31.0: DevTools actually returns the version of the underlying JS SDK (React is a wrapper SDK around the JS SDK). Back then the React and JS SDK version were out of sync, and React 0.31.0 embedded JS 0.21.0.