Update deprecated function

tfa.module

@@ -331,7 +331,7 @@ function tfa_login_submit($form, &$form_state) {
   }
   else {
     // Regenerate the session ID to prevent against session fixation attacks.
-    drupal_session_regenerate();
+    backdrop_session_regenerate();
 
     // Restart flood levels, session context, and TFA process.
     $identifier = config_get('user.flood', 'flood_uid_only') ? $account->uid : $account->uid . '-' . ip_address();