Replace deprecated String.prototype.substr() with String.prototype.slice() (#18796)

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 the slice() method which works similarily but isn't deprecated.

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/js/features/common-issue.js

@@ -20,7 +20,7 @@ export function initCommonIssue() {
     const issueIDs = $('.issue-checkbox').children('input:checked').map((_, el) => {
       return el.getAttribute('data-issue-id');
     }).get().join(',');
-    if (elementId === '0' && url.substr(-9) === '/assignee') {
+    if (elementId === '0' && url.slice(-9) === '/assignee') {
       elementId = '';
       action = 'clear';
     }