[3624003] Drupal 12 compatibility and Group 4.0 support

Issue: #3624003 Automated Drupal 12 compatibility fixes

Targets the new 2.1.x branch, which starts from 2.x with !28 (merged) merged.

Summary:

• Applies the Project Update Bot Rector changes: hooks move to OOP classes with LegacyHook shims, tests get PHPUnit attributes, and every info file allows ^12.
• The Group submodule accepts Group 4.x as well as Group 3.3.5+. Composer picks Group 3 on Drupal 10.3 and 11, and Group 4 on Drupal 12.
• Group tests enable Entity API and Flexible Permissions only on Group 3, because Group 4 dropped them.
• Fixes the list builder for Drupal 12, where the operations cacheability argument is required.
• Test doubles without expectations are now stubs, which silences PHPUnit 12 notices.
• Adds a next major (Drupal 12) CI lane.

Known issue: Group 4.0.0-alpha2 still reads $entity->original, which Drupal 12 removed, so saving a user crashes on Drupal 12 with Group installed. The fix is in Group #3631301 / MR !342. The Drupal 12 PHPUnit lanes are allowed to fail until Group releases it. With that fix applied locally, all tests pass on Drupal 12.0.0-beta2 with Group 4.