Show DevTools backend and frontend versions in UI (#23399)

This information can help with bug investigation for renderers (like React Native) that embed the DevTools backend into their source (separately from the DevTools frontend, which gets run by the user).

If the DevTools backend is too old to report a version, or if the version reported is the same as the frontend (as will be the case with the browser extension) then only a single version string will be shown, as before. If a different version is reported, then both will be shown separately.