Issue #3629646: Drupal 12 compatibility

Drupal 12 readiness, for #3629646.

Adds ^12 to core_version_requirement and fixes code that breaks on Drupal 12 (see the issue).

Not run on a real Drupal 12 install yet; needs review.

Backlink: https://github.com/YCloudYUSA/yusaopeny/pull/388 (Drupal 12 readiness of the YMCA Website Services distribution).

Note: core floor raised from >=9.4 to ^10.2 || ^11 || ^12 (attribute plugin discovery needs 10.2+).