Replace use of whitelist/blacklist in \Drupal\Core\Template classes and their tests

Problem/Motivation

Lets remove usage of "blacklist" and "whitelist" in \Drupal\Core\Template classes and their tests.

They are:

• An historic bad labelling of people
• Provide no context: "what is listed in them"?

Proposed resolution

TBD

Remaining tasks

1. Decide if we need BC.
2. Decide if we need to touch core/lib/Drupal/Core/Template/Attribute.php too. Only 'black-cat' and 'white-cat' and friends. Seems we could leave those.
3. Fix everything:
   • core/lib/Drupal/Core/Template/Loader/StringLoader.php - Comment only. See #3
   • core/lib/Drupal/Core/Template/TwigSandboxPolicy.php - Full of "whitelist":
     

       protected $whitelisted_methods;
       protected $whitelisted_prefixes;
       protected $whitelisted_classes;
     

4. Reviews.
5. RTBC.

User interface changes

None

API changes

None, only renaming some protected members.

Data model changes

@todo

Release notes snippet

@todo