cleanup: Replace deprecated String.prototype.substr()

The method is documented as deprecated:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr

Switch to the non-deprecated substring() method.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3234>

js/misc/dbusUtils.js

@@ -62,6 +62,6 @@ export function loadSubInterfaceXML(iface, ifaceFile) {
 
     return (
         xmlHeader +
-        xml.substr(ifaceStartIndex, ifaceEndIndex - ifaceStartIndex) +
+        xml.substring(ifaceStartIndex, ifaceEndIndex) +
         xmlFooter);
 }