Replace deprecated String.prototype.substr() (#8988)

String.prototype.substr() is deprecated (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr) so we replace it with slice() which works similarily but isn't deprecated.
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

modules/ui/fields/combo.js

@@ -434,7 +434,7 @@ export function uiFieldCombo(field, context) {
                     var v = tags[k];
                     if (!v || (typeof v === 'string' && v.toLowerCase() === 'no')) continue;
 
-                    var suffix = field.key ? k.substr(field.key.length) : k;
+                    var suffix = field.key ? k.slice(field.key.length) : k;
                     _multiData.push({
                         key: k,
                         value: displayValue(suffix),