Remove textual references to "Drupal 8"

Problem/Motivation

Core still contains many references to "Drupal 8" where it is no longer necessary to specify the version number.

E.g in core/lib/Drupal/Core/Render/theme.api.php

 * Drupal 8 uses the templating engine Twig. Twig offers developers a fast,
 * secure, and flexible method for building templates for Drupal 8 sites. Twig
 * also offers substantial usability improvements over PHPTemplate, and does
 * not require front-end developers to know PHP to build and manipulate Drupal
 * 8 themes.
 *
 * For further information on theming in Drupal 8 see
 * https://www.drupal.org/docs/develop/theming-drupal

Steps to reproduce

1. Search the codebase for "Drupal 8".
2. There are many references to Drupal 8 where there is no need to specify the core version.

Proposed resolution

Identify which textual references to Drupal 8 are no longer needed in the documentation.

Keep in mind that certain documentation relating to things like migration api might actually require core version specificity.

Leave those references as is.

Remaining tasks

Search for "Drupal 8" and determine on a case by case basis whether to remove the core version number from the documentation.

Release notes snippet