fix exception thrown when a user is low on printable codes

This error was shown in the browser:
```
SimpleSAML\Error\Error: UNHANDLEDEXCEPTION

Backtrace:
1 www/_include.php:20 (SimpleSAML_exception_handler)
0 [builtin] (N/A)
Caused by: TypeError: str_replace(): Argument #2 ($replace) must be of type array|string, int given
Backtrace:
7 lib/SimpleSAML/Locale/Translate.php:308 (str_replace)
6 lib/SimpleSAML/Locale/Translate.php:308 (SimpleSAML\Locale\Translate::t)
5 lib/SimpleSAML/XHTML/Template.php:914 (SimpleSAML\XHTML\Template::t)
4 modules/material/themes/material/mfa/low-on-backup-codes.php:32 (require)
3 lib/SimpleSAML/XHTML/Template.php:560 (SimpleSAML\XHTML\Template::show)
2 modules/mfa/www/low-on-backup-codes.php:35 (require)
1 lib/SimpleSAML/Module.php:266 (SimpleSAML\Module::process)
0 www/module.php:10 (N/A)
```

modules/mfa/lib/Auth/Process/Mfa.php

@@ -761,7 +761,7 @@ protected static function redirectToLowOnBackupCodesNag(
         $numBackupCodesRemaining
     ) {
         $state['employeeId'] = $employeeId;
-        $state['numBackupCodesRemaining'] = $numBackupCodesRemaining;
+        $state['numBackupCodesRemaining'] = (string)$numBackupCodesRemaining;
 
         $stateId = State::saveState($state, self::STAGE_SENT_TO_LOW_ON_BACKUP_CODES_NAG);
         $url = Module::getModuleURL('mfa/low-on-backup-codes.php');