WebUI: access attribute/property natively

It is now clearer to see what property is being accessed.
Previously mootools library would re-map attribute/property to another.

PR #21007.

src/webui/www/public/scripts/login.js

@@ -131,7 +131,7 @@ function submitLoginForm(event) {
 
 document.addEventListener("DOMContentLoaded", () => {
     const loginForm = document.getElementById("loginform");
-    loginForm.setAttribute("method", "POST");
+    loginForm.method = "POST";
     loginForm.addEventListener("submit", submitLoginForm);
 
     setupI18n();