Update config module to use the new batch builder

I've updated the summary to make the task clearer.

1. +++ b/core/modules/config/src/Form/ConfigSingleImportForm.php
   @@ -400,17 +402,16 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
   +          ->setFinishCallback('Drupal\config\Form\ConfigSingleImportForm::finishBatch')
   ...
   -          $batch['operations'][] = [[ConfigSync::class, 'processBatch'], [$config_importer, $sync_step]];
   

   We should be passing the class and method as the first parameter. This would be along the lines of ->setFinishCallback([ConfigSync::class, 'finishBatch'])

   The classes passed in do not match the currently used classes.

2. +++ b/core/modules/config/src/Form/ConfigSingleImportForm.php
   @@ -400,17 +402,16 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
   +          ->setTitle(new TranslatableMarkup('Importing configuration'))
   +          ->setInitMessage(new TranslatableMarkup('Starting configuration import.'))
   +          ->setProgressMessage(new TranslatableMarkup('Completed @current step of @total.'))
   +          ->setErrorMessage(new TranslatableMarkup('Configuration import has encountered an error.'));
   

   $this->t() should be used to make strings translatable.

3. +++ b/core/modules/config/src/Form/ConfigSync.php
   @@ -335,18 +337,17 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
   +          ->setFinishCallback('Drupal\config\Form\ConfigSync::finishBatch')
   ...
   +          $batch_builder->addOperation('Drupal\config\Form\ConfigSync::finishBatch', [$config_importer, $sync_step]);
   

   The class names do not match the currently used classes.

   Use the ['MyClass', 'myCallbackMethod'] format for the first parameter of the setFinishedCallback() and addOperation() methods.

4. +++ b/core/modules/config/src/Form/ConfigSync.php
   @@ -335,18 +337,17 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
   +          ->setTitle(new TranslatableMarkup('Synchronizing configuration'))
   +          ->setInitMessage(new TranslatableMarkup('Starting configuration synchronization.'))
   +          ->setProgressMessage(new TranslatableMarkup('Completed step @current of @total.'))
   +          ->setErrorMessage(new TranslatableMarkup('Configuration synchronization has encountered an error.'))
   

   Please use $this->t() for translatable text.

@James.Shee, when in class methods you use $this->t() and in functions (outside of classes) t() is used.

From the StringTranslationTrait documentation:

Using this trait will add t() and formatPlural() methods to the class. These must be used for every translatable string, ...

This helps with code injection (swapping the code of equivalent interface). This is most commonly used for unit tests.