Fix php8.2 issue related to deprecated function utf8_decode

Closes #3424960

theme/views_data_export.theme.inc

@@ -113,7 +113,8 @@ function template_preprocess_views_data_export_csv_header(&$vars) {
     if (!empty($vars['options']['encoding']) && function_exists('iconv')) {
       switch($vars['options']['encoding']) {
         case 'utf8_decode':
-          $converted = utf8_decode($output);
+          $converted = function_exists('mb_convert_encoding') ?
+            mb_convert_encoding($output, 'ISO-8859-1', 'UTF-8') : $output;
           break;
         default:
           $converted = iconv("UTF-8", $vars['options']['encoding'], $output);
@@ -160,7 +161,8 @@ function template_preprocess_views_data_export_csv_body(&$vars) {
       if (!empty($vars['options']['encoding']) && function_exists('iconv')) {
         switch($vars['options']['encoding']) {
           case 'utf8_decode':
-            $converted = utf8_decode($output);
+            $converted = function_exists('mb_convert_encoding') ?
+              mb_convert_encoding($output, 'ISO-8859-1', 'UTF-8') : $output;
             break;
           default:
             $converted = iconv("UTF-8", $vars['options']['encoding'], $output);