[SITE-1855] Declare Drupal 11 compatibility

[greg-1-anderson]

This PR is based on the patch submitted via the project-update-bot on drupal.org.

Pantheon Advanced Page Cache does the following things:

• It registers a cache tags invalidator (Drupal\Core\Cache\CacheTagsInvalidatorInterface)
• It registers an event subscriber (Symfony\Component\EventDispatcher\EventSubscriberInterface)
• It implements hook_ENTITY_TYPE_update()

We know the following:

• CacheTagsInvalidatorInterface did not change between Drupal 10.x and Drupal 11.x
• EventSubscriberInterface did not change between Symfony 6.4 and 7
• The hook and entity system did not change in Drupal 11

This gives us some level of confidence that this level of change should be sufficient for Drupal 11 compatibility, and, in fact, this module does install on a Drupal 11 site.

Tests are currently failing, though, and it would be good to have some level of actual testing done here before assuming we are done.

[greg-1-anderson]

Got the tests running again on Drupal 9 and Drupal 10, but the Drupal 11 tests are failing for reasons I can't quite explain (yet).

[greg-1-anderson]

Continued in #58. Force pushing breaks the drupal.org update; re-opening on a new branch rather than manually force-pushing on the drupal.org repository.