Use the array of validators in the correct format depending on the version of Drupal

Use the array of validators in the correct format depending on the version of Drupal.

In the edge case where a site is using a pre-release version of Drupal 10.2.0 where the old format is deprecated, they will get deprecation errors. That should not be a problem.

Closes #3480275

src/Form/MigrateSourceUiForm.php

@@ -107,8 +107,8 @@ class MigrateSourceUiForm extends FormBase {
       '#type' => 'file',
       '#title' => $this->t('Upload the source file'),
       '#upload_validators' => [
-        'file_validate_extensions' => [
-          'json csv xml',
+        'FileExtension' => [
+          'extensions' => 'json csv xml',
         ],
       ],
     ];
@@ -135,7 +135,7 @@ class MigrateSourceUiForm extends FormBase {
     $migrationInstance = $this->pluginManagerMigration->createStubMigration($definition);
     $extension = $this->getFileExtensionSupported($migrationInstance);
 
-    $validators = ['file_validate_extensions' => [$extension]];
+    $validators = ['FileExtension' => ['extensions' => $extension]];
     // Check to see if a specific file temp directory is configured. If not,
     // default the value to FALSE, which will instruct file_save_upload() to
     // use Drupal's temporary files scheme.