Use OOPs way to fetch \Drupal::VERSION in Class

Problem/Motivation

In the class Fast404.php we are fetching the Drupal version using \Drupal::VERSION
It will be better to use the Object oriented way to get the Drupal Version.

This line of code was introduced to overcome the css break caused by Aggregation.

Proposed resolution

We can use the @update.manager Service instead and fetch the Current Drupal Version, and pass it to the extensionCheck() function where the Version comparison is required.