Replace deprecated MigrateSkipProcessException

This module uses MigrateSkipProcessException:

./tests/src/Unit/process/SkipOn404Test.php:5:use Drupal\migrate\MigrateSkipProcessException;
./tests/src/Unit/process/SkipOn404Test.php:31:    $this->expectException(MigrateSkipProcessException::class);
./src/Plugin/migrate/process/SkipOn404.php:5:use Drupal\migrate\MigrateSkipProcessException;
./src/Plugin/migrate/process/SkipOn404.php:85:      throw new MigrateSkipProcessException();

It is deprecated as of Drupal core 10.3: https://www.drupal.org/node/3414511 →

The new way to deal with those is to implement the isPipelineStopped() method: https://www.drupal.org/node/3247580 →