WebUI: Replace Mootools class list manipulation methods

All `addClass()`, `removeClass()` and `hasClass()` instances were changed to use `classList` equivalent:
https://developer.mozilla.org/en-US/docs/Web/API/Element/classList

PR #21946.

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Co-authored-by: Chocobo1 <Chocobo1@users.noreply.github.com>

src/webui/www/private/scripts/search.js

@@ -238,7 +238,7 @@ window.qBittorrent.Search ??= (() => {
             return;
 
         const searchId = getSearchIdFromTab(tab);
-        const isTabSelected = tab.hasClass("selected");
+        const isTabSelected = tab.classList.contains("selected");
         const newTabToSelect = isTabSelected ? (tab.nextSibling || tab.previousSibling) : null;
 
         const currentSearchId = getSelectedSearchId();