cache contexts should use `list<string>` everywhere

Problem/Motivation

#3468502: sebastianbergmann/comparator:5.0.2 Introduces (for Drupal) breaking changes was necessary because sebastianbergmann/comparator made changes.

Those changes were very superficial (only type annotations in docblocks that go beyond PHP's native yet simplistic array in actual code), but enabled sebastianbergmann/comparator to continue to work in unit tests.

However, projects that are developed with a higher PHPStan level than core — for example, Canvas is developed using level 8 — run into a remaining failure:

  102    Parameter #1 ...$cache_contexts of static method Drupal\Core\Cache\Cache::mergeContexts() expects list<str  
         ing>, array<string> given.                                                                                  
         🪪  argument.type                                                                                           
         💡  array<string> might not be a list.                                                                      
  102    Parameter #2 ...$cache_contexts of static method Drupal\Core\Cache\Cache::mergeContexts() expects list<str  
         ing>, array<string> given.                                                                                  
         🪪  argument.type                                                                                           
         💡  array<string> might not be a list.       

— https://git.drupalcode.org/project/canvas/-/jobs/7375701

because

Steps to reproduce

PHPStan level 8, and something like:

Cache::mergeContexts([], User::load(1));

(or really, anything that implements CacheableDependencyInterface)

Proposed resolution

Adjust docs for \Drupal\Core\Cache\CacheableDependencyInterface::getCacheContexts()

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet